I'm not particularly keen on a non-glyph-related means of altering/removing diagram and effect colors. There will be some glyph that do things like hide the diagram from all but the person who drew it, but stuff like that is for Arcanists who will be able to draw diagrams with more than just two glyphs in them. Otherwise, ff you're worried about people knowing you drew any particular diagram on the ground, I'd recommend drawing them when nobody else is around and not leaving them for people to see.
Acarin wrote:
It seems odd that our chalk magically changes color as we draw.
Try LOOKing at the chalk sometime. Combine what you see there with the fact that you're using it to draw occult diagrams, and at the very least, "odd" things should be expected. It's working as intended.
I do like the idea of an ability to allow drawing hidden circles in the open or while stealthed though
See the above for hidden circles, though that's for Arcanists only. Not being able to draw occult diagrams while hiding is intended. I see it as a bit too involved and taking up too much room to be done while also trying to remain positioned/motionless/crouched/whatever-else in stealth.
Sneaky sketchy diagram-drawing is best done in out-of-the-way places where one won't be spotted. And while we're on the subject: What's the particular reason for not wanting people to see your diagrams or their color?