Colors are numbers
Colors in p5 are mixed from red, green, and blue, each from 0 to 255. background(r, g, b) fills the whole canvas. fill(r, g, b) sets the color of shapes you draw after it. stroke(r, g, b) sets their outline.
fill(255, 0, 0)→ redfill(255, 204, 0)→ yellowfill(0)→ black (a single number is a shade of gray from 0–255)noStroke()removes the outline;noFill()removes the fill
NOTE
Order matters: set fill(...) before the shape it should color. A fill stays active until you change it.