You could do something like this:
for(i! = 0; i!<=255; i!+=25){
ambientred! = i!
ambientgreen! = i!
ambientblue! = i!
forceredraw()
}
(Note: Didn't test that trick in TK3 yet.)
Basically just redraws the board lighter and lighter repeatedly. You can increase the 25 in 'i!+=25' to make it go faster if it's too slow.