I've had to handle a lot of paint on front sights, roll stamps, serial numbers etc.
Just wipe the gun down with clean cotton rag (old T-shirt) to get rid of oils. grease, etc. Remove wooden or plastic grips.
The I use regular cotton swabs dipped into a small bottle/can of acetone, liberally soak the painted areas. Let soak for a few minutes (5 will usually do it...10 if really old and stubborn). Then just use an old nylon toothbrush and it comes right off.
Wipe down again, oil and final wipe as you do after normal cleaning and done...done...done.
I've cleaned white paint, neon red, orange and all sorts of high vis paints....usually they are just Testor's Model paints.
Never any harm to bluing, or to stainless finishes, and I've done a lot of old rifles, Colts and Smiths over the years. The white paint was popular when 1st internet listings started coming around.