Hi,
I’m unable to paint my walls white. I can paint some of them, but not the ones that are blue/grey.
Please send help 
When you model:
- Turn off AO (ambient occlusion);
- Turn off Shadows;
- Orient the faces correctly;
- Use groups and tags.
The ones that are blue and grey are actually back to front and you are seeing the back side of them - it’s probably a good time to turn those around as it causes problems in other places.
FRONT AND BACK SIDES OF FACES in SketchUp and Why They Matter