This is not solution, but I did have the same problem, with max 6. Some times when I clicked on a color it lowered its values by 0.3 - 0.5. If I clicked many times, it gradually when full black.

It hasn't happened yet with Max 7. Maybe you have opacity in the left panel?

Too bad its really colorpicker choosing a slightly different color. Tried it with some more colors. Funnily the colorpicker itself finds the color but gives out a wrong color inside Max/Msp. http://i.imgur.com/U3XHjLo.png

Check if the difference in the colors (the RGBA values) is consistent, maybe have a "correction offset". I know its frustrating, I drove me crazy when making GUIs in Max 6. Patience!!

Nik