It's amazing what you can do with the simple box command!
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Value# = 100
IncValue# = 0.5
Value2# = 255
IncValue2# = -0.25
Value3# = 150
IncValue3# = -0.10
Value4# = 200
IncValue4# = 0.5
do
inc Value#,IncValue#
inc Value2#,IncValue2#
inc Value3#,IncValue3#
inc Value4#,IncValue4#
if Value# >= 255 or Value# <= 100 then IncValue# = IncValue# * -1
if Value2# >= 255 or Value2# <= 100 then IncValue2# = IncValue2# * -1
if Value3# >= 255 or Value3# <= 100 then IncValue3# = IncValue3# * -1
if Value4# >= 255 or Value4# <= 100 then IncValue4# = IncValue4# * -1
box 0,0,640,480,rgb(Value#,0,0),rgb(0,0,Value2#),rgb(0,Value3#,0),rgb(0,Value4#,Value4#)
loop
bjdgames