Not the small tube, not the great big one either, but it depends how opaque you want it. I have mixed quite a lot of paint in, most of a big tube of white and Mars black for the hull, and it does not mess up varnish. I actually think it makes it less apt to crack.

Mix spoonfuls of varnish into the paint until you develop a smooth slurry, then mix that into cups of varnish. Go the other way and you will chase bits of paint forever.