fill(1, 0, 0, .5)
# draw a rect
rect(10, 10, 200, 980)

# only set a gray value
fill(0)
# draw a rect
rect(200, 10, 200, 980)

# only set a gray value with an alpha
fill(0, .5)
# draw a rect
rect(400, 10, 200, 980)

# set rgb with no alpha
fill(1, 0, 0)
# draw a rect
rect(600, 10, 200, 980)

# set rgb with an alpha value
fill(1, 0, 0, .5)
# draw a rect
rect(800, 10, 190, 980)