[Alaya]

I don't think we have chemicals that react with water and change it's color (and I say this as a Chemistry student).

Normally for a compounds to have a color, it must form some kind of complex with transition metal, or form a poly-conjugated system, which normally water fail to do so. It's possible to have some kind of metal complex that give red color (Iron thiocyanate as an example) but normally those kind of complex will be presented as a solid crystal so it not practical to "paint" it on the door.

For long conjugated system, normally they don't dissolve in water and the reaction with water normally requires acid as a catalyst so it's quite impractical too.

EDIT: Well, there are some elements that have color (Chlorine, Bromine, Iodine) but they are natural color without reacting with anything.

EDIT2: However, I won't deny the possibility of some compounds that I don't know, or are only known by specialists in dye and paint. It's possible that such a thing exist, but very unlikely.