примерно так
Graphics3D 800,600,32,1
SetBuffer BackBuffer()
SeedRnd MilliSecs()
For i=1 To 100
Color Rnd(256),Rnd(256),Rnd(256)
Rect Rnd(0,600),Rnd(0,400),Rnd(0,400),Rnd(0,400)
Next
CopyRect(0,0,800,600,0,0,BackBuffer(),FrontBuffer())
While Not KeyDown(1)
Flip
Wend
fadescreen(4000)
Function fadescreen(time)
dt#=time/255
v=0
While time>0
For i=0 To 255
SetGamma i,i,i,i-v,i-v,i-v
Next
UpdateGamma()
time=time-dt
v=v+1
Wend
End Function