Need some help with javascript

I have a word search puzzle in PDF format that I need to make interactive. The only thing so far that seems to work is adding buttons to each letter. My thought is once the user finds the word they can select each letter (button) to spell out the word. 

My dilemma is that I need the colors of each button to be transparent enough for the letters to show through. Plus I would like for the colors to toggle between a blue (selected), green (correct) and transparent (starting point). I saw a previous post about javascript for buttons changing colors, but the colors weren't light enough for my letters to show through.