Dye in Emulsion Paint

[gumboil]

I've just finished painting the ceiling with one of those white emulsion paints that contains a pink/lilac dye for you to see where you've painted. The dye colour fades completely in a short time.
Does anyone know the name of the dye or type of dye used for this purpose and how it works? There's no information about it on the paint container.
I think these paints are a wonderful invention!

[Pressgang1]

You know, I've been wondering this too, and then I remembered there is a dye called phenolpthalein which is pink in alkali and clear in acid/neutral conditions (thank you HNC chemistry!)

So if you put the phenolphthalein into an alkali medium like sodium hydroxide, as the paint dried, the dye would react with the CO2 in the air and go less and less alkali and eventually neutral ie clear.

Just a guess mind, but it could work ... hmmmmm