I figured out my problem. I've got it to work. Now it look like this:
to mycolors
cs
pd
repeat 4 [fd 200 rt 90]
pu
rt 45
fd 20
make "red 0
make "green 0
make "blue 0
while [:red<257] [
while [:green<257] [
while [:blue<257] [
setfloodcolor (list :red :green :blue)
fill
make "blue :blue+1
]
make "blue 0
make "green :green+1
]
make "blue 0
make "green 0
make "red :red+1
]
end