color changing chemical

I dont know if this helps at all but i found it online:

Self-indicating silica gel is the only thing I know which changes
color depending on whether is wet (pink) or dry (blue). It's just
regular silica gel with a cobalt chloride indicator added, which is a
very toxic stuff by the way.
There are other less dangerous silicas (check here 
http://www.geejaychemicals.co.uk/cobaltchloride.htm  )
I've worked with these substances for years and the process can be
reversed anytime and as often as you want, but I really don't think
they can be used as additives in a paint.

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