that’s “z-fighting” You have two faces in the same plane and same place. Figure out why you have two faces and get rid of one. Looks like you have a front face and a back face (bluish color), so you want to keep the front face --in the direction that is the natural outside face of the object. If that is a ceiling, it would be the face that is facing down, etc. Get the white face facing out on on the surface of the object and paint that. Keep the bluish face hidden usually, inside the object.
Thanks I got rid of the faces, however I need to use the bucket on the area inside the walls of the floor plan without colouring anything else. When I use the bucket it does this, lines that I want to stay black turn grey as well. I guess I’ve gone wrong in the grouping or something somewhere.
When you paint a group, everything in it that is the “Default Color”, takes on that color. It can be confusing because, out of the box, the Default Color is white on the front side but it’s not the same as the actual color white. It’s kind of like a variable instead of a specific color. If you don’t want everything in the group taking on the same color, it’s better to enter the group and paint each face individually the way you want it. Edges can have colors, but under styles you can choose to turn that off and make all edges the same color.