setc -Technical information-  
When using decimal numbers, MicroWorlds tries to find the best color match.

For example:
setc 19.9
You may assume that this should be a very dark red. Yet it becomes black because the closest color to very dark red is black.

Try this example in 16 bit color mode:
seth 45
pd
repeat 100 [setc color + 0.1 fd 20]