I'm assuming this would be a good spot to ask this. I'm looking for a solution that is clear initially but will change a very dark colour and probably stain when exposed to either direct sunlight (at least take 5 or 10 minutes) or rainwater, or cold temperatures (about minus 5C to minus 15C). Basically this solution must be able to be absorbed or coated onto a material, such as a cotton or polyester and mustn't damage or stain said material until one of the above conditions is met. The solution must also be relatively easily obtainable or ‘makable’ by me. Does anyone have any ideas?