Try the following code:
Color 1, 0, 0
Fill
Repeat 6
Forward 80
Right 60
The three numbers after color must be between 0 and 1. They represent the intensity of red, green and blue. (0 means none, 1 means alot)
To clear the screen and change the color use a line such as:
Clear 1, 0, 1
When this is used at the beginning of a program when the screen is empty it just changes the background color.
Instead of typing in 3 numbers for the color you can click on the color picker button which just to the left of the Go button. Select any color on the pallet 
or create a custom color and use that.