A brief fad in Victorian England, well after the years in which letters were sealed with wax by necessity. The color indicated the subject matter. Blue was for romantic messages, white for invitations, black for mourning a death, green for casual correspondence, red for formal and business correspondence, and pink for letters of praise.
I imagine it was a brief fad because you don't want to openly display the subject matter of confidential correspondence, which category romantic messages often fall into.
Imagine having to entertain Lady Asquith (whom you refer to as "Lady Ass Quit" among your friends behind closed doors), smiling politely as possible and allaying all suspicion that you are intending to refrain from calling upon her ever again. You are handed a letter by the butler, sealed with white wax. Well, now Lady Asquith wants to know where you have been invited, and if she might receive a similar invitation, and this awful color code has caused her to have further reason to be in your life when you really needed some space and time away from her. And if she fails to receive such an envelope, she will become resentful and envious, and send you letters sealed in green that are full of very polite and pointed references to how awful it must be to be so popular. And So your entire teatime has become awkward.
Now imagine being with your friends in the smaller of your two parlors, and you receive a letter sealed with blue wax, which is absolutely terrible timing, you must have a private word with the butler not to deliver such messages when you are in polite company, and you strain to call the company of your friends polite when it comes to romance because their whole opinion of the subject is, frankly, childish, such that if they know someone (you) has received a lover's letter, they start to coo about who the lover boy could be. They demand to know. They ask for the letter for to read it themselves. This is a serious problem, because the message is from Lady Asquith (whom you have realized, in the months since that disturbing and not entirely diastrous ball, has an ass that WON'T quit, and besides which she is the only person besides you who understands what it is like to have your father die a week before your Debutante Ball, thereby forcing you to have a stiff upper lip during your entire presentation, which posture completely breaks down in the middle of your first dance where everyone is watching, and the poor sap who was dancing with you thinks it's his fault, and sends you a heartfelt letter of apology later, which you never did answer, because how could you explain such a thing to a complete stranger?
And Lady Asquith is the only person you have ever met, besides your mother, who offers kind and encouraging words regarding the matter, such that you have begun to wonder who your actual friends are, and why you continue to gather for teatime with these frankly rude and shallow ladies that you call your friends; possibly it is out of inertia, because at the very least they were the ones who bothered to stick around in the years after the Debutante Ball when you barely called upon anyone and let all your old friends drift away, as if, despite their shallow talk and immature ideas of romance, they have depths that you had not considered carefully beforehand, like Charlotte, who forgives much and gives much (even to beggars), or Veronica, whose silent gaze seems to hold great wisdom, or Mary Anne, who continues to spin yarn despite her mother telling her that work is for peasant girls, or Elizabeth, who has taken entirely upon herself the job of revitalizing the gardens of her estate, betimes even getting down into the dirt herself to haul out weeds. And Elizabeth is nearly as pretty as lady Asquith, indeed, which you had, once upon a time, considered more often, but put the full implications of such thoughts aside, calling them absurd, obscene, insane, a product of brain fever perhaps.
Lady Asquith is oh so pretty, even when she lets her hair down, and perhaps it is because you are drawn to her that you are willing to see her qualities in a different light, and what seemed like pig-headed stubbornness now appears to be bold resolution, and what always felt like unsolicited advice, now that you are willing to listen, can be considered solicited, even if you didn't ask, and what appeared to be an annoying tendency to finish your sentences for you now feels like she is always a step ahead of you, the clever girl. Perhaps you forgive her too much now, perhaps you favor her too much, and should really stick to the friends you know, even if you think they are shallow. Or should you? Perhaps it's time to invite Lady Asquith into your circle, and yet none of your friends would accept this, for they always become icily polite in her company. It becomes her versus them, and it may be that you are forced to choose.)
Oh, goodness, The dilemma that comes of this business, and now that your friends know you have a lover, they insist on knowing who it is, and the first thing you think is that the dratted Lady Asquith should have sent a letter with a green seal, a red seal, a white seal, anything other than a blue seal, the absolute fool, she knows this color-coding business is risky given the frankly scandalous nature of your correspondence, and perhaps you shall not speak to her for a few months as recompense. And the second thing that goes through your mind is that as long as you stay in this room you may yet be found out by your friends, and even if they are not immediately scandalized they will probably blab to someone who would be, such that you will undoubtedly be examined by a doctor, and the least that can happen is he forces you to stay in bed and rest your brain for an entire month, and the worst is that you are hauled off to a mental asylum for perversion, which is a terrible place where the floors are hard and you have to stay behind bars and they poke and prod you, and besides which, it's for poor people, so it would be more kind to simply be locked in the attic and forgotten about, and whatever else happens your association with Lady Asquith would come to an abrupt end, never to be restarted again, for her mother would surely pack her off to her relatives in Barbados as she has been threatening.
So as your face turns red you run out of the room, dropping the letter behind you, and you look rather comic as you spin around and rush back to it, except that Veronica is already handing it to you, so you look even more foolish, and then you run out of the room, into a corridor that only the servants use now and then, and stop there to catch your breath and calm your nerves, only to see Veronica and Mary Anne approaching, and you think, My God this is it, my secret is out, I'm ruined.
Which causes you to wonder when Veronica kisses you once on each cheek, because she only does that when wants to tell you something important, because she wants to be sure she has your full attention, and this time around is no different, save for the gravity of the message, for she says, very quietly but firmly, you are not the only one. I apologize for failing to tell you sooner.
And Mary Anne says, just as quietly and just as firmly, your friends are with you forever, no matter what happens.
And they lead you back into the parlor, and you sit down, not saying a word, and not needing to, for the look on each woman's face says enough: each of them knows, and will never discuss the matter outside this circle.
Goodness, this has been a wonderful and terrifying day, even moreso than the ball so many months ago, where Lady Asquith kissed your hand and told you she had a great number of things to discuss with you later, leaving you thoroughly confused and strangely nervous, the source of which is much easier to understand now.
You see what comes of doing a color code for the subject of sealed correspondence? This day would have been perfectly peaceful if that damned envelope had never arrived.
You receive a letter sealed with pink wax a few days later, from Lady Asquith, congratulating you on being more honest with your friends, in a way that made your bond all the stronger. She hopes to meet them someday soon in a manner where they can all start out on the right foot this time. And you think, what the fuck, did she plan this entire thing with Veronica behind my back? So you throw out every color of sealing wax you have except red, because this awful color coding business has nearly caused you a nervous breakdown. Lady Asquith will have to be content with being surprised by the subject matter of your letters from now on. So there.
That's why the fad was brief.