For people having problems with dithering of .gif images, I found that most of the 256 colours in the .gif palette are based on multiples of 51 for the RBG values i.e. (0, 51, 102, 153, 204 and 255.) Any colour with these values will not break down into other colous when saved as a .gif. So as long as people only use those colours, the picture will turn out correct.
There is a plug-in that I saw that will change colours in a picture to the colour closest in a defined palette, Which should work, but I don't remember what it was called.
Having six values for three variables gives only 216 possible colours; I don't know what the other 40 could be.
--Nekura
