Well, due to dust, they won't stay white for long. Good thing about snow / glaciers is they are repainted every week! Doing it for entire mountains manually would be unrealistic.

At this point, if you want to play with geoengineering, dispersing particulates into upper atmosphere and placing Sun shields in high orbit would probably be a better idea.

Using hot air balloons as sun shades would be much cheaper than satellites in high orbit. And balloons would give more flexibility as they can be used to heat the air above clouds and disperse them when needed. But most importantly they can be used to improve actual weather, so even the people that are not interested in abstract climate change would support them.

Check how big Greenland and the Arctic are. You'd need a lot of paint.