Shubert,

Having selected your object, use the 'Color' mode (Not Stamp or Mix) in the Chroma key window. The cursor in the previe window changes to a little eye dropper and cross hairs and you can drag out a rectangle across the color to be transparent. Holding down the [Ctrl] key you can add to your color selection in a way, but you can't pick red and then also just yellow. You get the red and yellow but also all the shades in between.

Note: In the progam settings window [Y] key, the 'Display options' tab has a video mode drop down menu. Some of thesse may not allow transparency color picking as the preview image is held I guess in video card RAM and not in the screen memory bytes accesable by a click and drag.

Regards,

Carl