[Zededin]

I am looking for a compound that is most reactive (and preferably changes color) when exposed to even minute amount of water particles at a particular location on a surface.  I have already tried a Cobalt Chromium paper that changes from pink to blue when the humidity in the air is increased.  However, the sensitivity of the paper that I could acquire is low and changes in color takes a long time.  Thank you in advance.

[Donaldson Tan]

Perhaps adding transition metal salts to the hygroscopic NaOH pellets might yield something interesting. NaOH helps to absorb moisture, the transition metal ion changes colour in the presence of water.. It's just a conjecture, but why not? I think there's no harm making a suggestion

 

[AWK]

Perhaps adding transition metal salts to the hygroscopic NaOH pellets might yield something interesting

But all color cations form hydroxides insoluble in water.

[Donaldson Tan]

Is there any known hygroscopic transition metal compounds that may do the trick, other than Cobalt?

[hmx9123]

There's a good number of inorganic compounds that change color (and usually catch on fire) when exposed to even small amounts of water.  Problem is, they're usually pryophoric as well.  I'm guessing that's not what you're looking for.