Charcoal affects color

Charcoal affects color
Further clarification: I use Liquitex medium viscosity (now called "soft body") paints.

Hmm, that means that there is something water or solvent soluble in the charcoal. Do you use compressed charcoal pencil or those peel and use sticks? Maybe using a soft graphite.
I do watercolors mostly so I usually sketch with a colored pencil thats watersoluble. I often use a complementary color in the pencil cause it makes the watercolor pop out when the complement is just hinted along the drawing margins. Try using a blue pencil, it may afford you some neat effects

Clarity
Hi

I had the same problem ... My solution was to seal the canvas before I applied the charcoal .... I spray with sealant ... or I paint over the canvas ... .. I've heard wood glue mixed with water also works but have not tried this myself

Canvas is very porous so it absorbs just about everything and when you apply something on top it naturally seeps back up through especially paint ..

The problem is not in with the charcoal it's with the canvas ... seal it and you won't have a problem ...

Charcoal affects color
I always put a coat of background color - usually Titanium White - on the canvas. Then I use vine charcoal to draw, and remove as much of it as possible so that only faint outlines appear on the canvas. But when painted over with yellow they still pick up that greenish hue which is quite difficult to cover.

I prefer charcoal to even the softest graphite because I can reduce the drawing to very very light lines.

The charcoal is Winsor No. 1 Vine. There's no other info on the box. The sticks are uneven thin twigs. They certainly appear to be pure charcoal, with no tint or taint of any other material.