How do I build a thermochromic thermometer?
We calculate the $K_c$ values. Ratios that give $K_c$ greater than 1 are product favored and will turn from blue to pink at low temps. The opposite is true for the ratios that give $K_c$ values less than 1. These are reactant favored, and will turn from pink to blue at the low temp. First, look to see what is chaging color most drastically, meaning the reaction is proceding to a greater extent ($K_c$ value a lot greater than or less than 1), then look at the cost. If it's too expensive, a thermometer that is less effective but still OK, may be better.

Overall, think of a thermometer that you stick on the side of a fishtank (sort of).