Walls are dark instead of lighter

I just begun the game (I never played before today). I already took a look at both posts I found about this, but none of the solutions work. Basically, when I paint a wall yellow (for example), it's sorta yellow, but very dark - there are 2 lamps and 4 windows, and it's during the day. My GPU is more than capable of playing the game (settings are maxed out). I tried the roof thing, but when I lower the roof, it lowers the wall (not that I had a roof at first lol (had a ceiling, not a roof though, but I added the roof to see if that would make a difference). I legit have a rectangle, without any breaks. Nothing more than that.