Why does every time i try to draw over something else it change to white?

everytime i have tried to draw over another line of the same colour, it changes to white.
This is very inconvienent as i love to draw and i really need this to stop.
I often over lap lines and i specialise in pixel art.
I spent about ten minutes of my time just trying to draw that simple face and about 15 trying to figure out the problem, and i think ive found what it is.
After some clicking aroun i found the tab for Colour Picker (background colour) and i think thats my problem.
It hasnt done this before and i desperately need this fixed asap.
If anyone has any fixes thatd be so so so great.
If i find a fix for those having the same problem as me i will keep this updated.
Thank you.

