I think is something we can debate about. In my opinion it doesn't really matter which color you choose as a background color. It all depends on the image itself. If you have one with a lot of purple in it, then magenta isn't the color either...

Anyway, I discovered that selecting a transparency color doesn't always work at diferent color depths.

When selecting the color by looking at pixel[x,y] you might want to try typecasting it to a word

[pascal]if (word(YourSurface.Pixels[X,Y]) = 65535) then showmessage('you have clicked on a white pixel');[/pascal]