Drawing coloured bitmaps on a monochrome image

I can't understand what's happening here and would appreciate some help.
I'm trying to draw coloured (red) crosses on the canvas of a black and
white image loaded into a TImage component, but the crosses always came
out black - until I discovered by chance that if I swap the image out to
a temporary bitmap object (before I've drawn any crosses) and swap it
back, my crosses appear red. So this works for me, but I have no idea
why! Can anyone suggest why or a better way?


Alan Hale