Hi, I played with colors and found out, that arithmetical operations with colors are completely broken. https://pharo.fogbugz.com/f/cases/15188/Color-ariphmetical-operations-are-br... Result is not a valid Color - just garbage. It happens because alpha is not being initialized and stays nil. It seems that operation were implemented before alpha support was added. So I propose the following solution: Color red - Color red = Color transparent (Color white alpha: 1) - (Color white alpha: 0.5) = (Color white alpha:
0.5)
R,G,B are subtracted as before. Alpha channels are subtracted too. Color red + Color blue = Color magenta Color red + Color transparent = Color red (Color white alpha: 0.5) + (Color white alpha: 0.5) = (Color white alpha: 1)
R,G,B are added as before. Alpha channels are added too. Multiplication and division by number doesn't change alpha channel. Cheers, Alex