You can mix red, green, and blue to create colors. As it was with grayscale, each component gets a value between 0 and 255.
fill(255, 255, 0); Fill with three parameters sets a red-green-blue color. This one is yellow.
fill(0, 255, 255, 200); Fill with four parameters sets a RGB-color with transparency. This one is a little bit tansparent cyan.
background(255,0,0); Covers the whole canvas with the specified color. This one is red.
You can turn off fill and stroke with
strokeWeight(1); sets the thikness of outlines.
strokeWeight to color the sketch below: