Hello.
This is how my floor plan looks. (I am a sketchup up beginner so don’t mind mistakes and design fails, I just started studying interior design).
I would like to change the texture of the floors, but as you can see the floors are grey… I erased the floors because I thought it didn’t matter and now I cant add color or texutre. Do you know how to change this?
Draw a rectangle from top-left corner to bottom-right corner of the house (make it a group).
But most certainly before that you have to make groups of what you have already drawn: the walls, the furniture, the stairs, etc …
Start to learn the basics from here: https://learn.sketchup.com/track/sketchup-fundamentals-part-1
If you want separate areas to have different flooring’s you will hive to make separate rectangles (geometric shapes)
Thank you but that doesnt work. I can only paint blue surfaces (and I know how to do it) but not the gray ones. So my question is: how do I turn gray surfaces to blue? I erased it and don’t know how to undo it. 
I think you are misunderstanding one of the basics of the software, the ‘gray surfaces’ you mention are simply a lack of face, you are looking at the background. The blue faces are the back faces of actual geometry.
What did you delete? Basically you need to draw in the faces you need.
You could add your model so we could give you specific answers.
I’d recommend spending some time doing the tutorial in the campus to get an understanding of the fundamentals.
