How do you remove red window paint from white painted cement walls without taking off the white paint?

If the white paint is well-bonded to the surface, and the red is very fresh, a lot of times you can use a sharp razor blade to peel the red paint off the white. If the red is stuck on there, I would use a sanding block to remove as much red paint as I could without damaging much of the white paint, then put a coat of white paint over the area. Next time, get a newspaper and tape it to the white walls to protect the building.