I'm not sure if that will give you what you need, but for example:
LINE (0, 0)-(639, 479), 13, BF
FOR n = 1 TO 1000
x = FIX(RND * 640)
y = FIX(RND * 480)
Radius = FIX(RND * 100)
Colr = FIX(RND * 16)
CIRCLE (x, y), Radius, Colr
EightBIT 0, 0, 639, 479, "Circ8bit"
This will generate an 8-bit bitmap of a SCREEN 12 image, wherein only the first 16 colors are used. The real problem is if you want to convert back to a lower bit-value after having worked on an image. That's where you get all the reinterpretation of colors as halftones, etc.. Converting to 24-bit bitmaps in a Paint program isn't a problem, of course, but once again, converting down always is.
There's also a problem with mspaint when it comes to conversions. It's probably the worst of all applications at it -- some of the results are horrendous. You should really grab a copy of some free paint app off the net.