The following videos demonstrate a few different ways to color your cold porcelain. The crafters in these videos are using markers, acrylic paints & make-up. You can also use oil paints, paste colorants (some made for cake decorating & some made specially for cold porcelain), pastel chalks and just about anything else that will transfer color!
If you do not color your cold porcelain paste at all, it will dry a translucent off-white. If you want a white-white, you'll need to add a whitener to the recipe or color the paste white. Making your cold porcelain white from the beginning (by adding whitener to recipe) will also make any colors you add later brighter and cleaner looking. The whitener I use is Wilton White-White ...popular as an icing color for cake decorating and readily available in my area.