Simple, change DayNight to be an integer like so ..
While Not KeyDown(1) DayNight = ( DayNight + 1 ) Mod 400 COLORinterpolate( DayNight / 100.0 , 64,255,64, 64,64,255, 255,64,64, 255,64,255 ) AmbientLight aR, aG, aB RenderWorld() Text 0,0, ar + " "+ ag + " " + ab Flip Wend