you have to correctly texture your object with an alpha map, leaving the parts which will change color open and then use set object diffuse command to change color of only a part of the object.
Here is an example:
Use attached image
Make Object Plain 1, 100, 100
Load Image "Mask.bmp", 1
`Texture Object 1, 1
Set object diffuse 1, rgb(255,0,0)
Set Blend Mapping On 1, 1, 15
Set Blend Mapping On 1, 1, 5 `<--- you can rem this out
Sync On
Do
a = a + 1
If a => 255 Then a = 0
Set object diffuse 1, Rgb(a,0,0)
Sync
Loop
Here is another way without blend mapping:
Make Object Plain 1, 100, 100
Load Image "Mask.bmp", 1
Texture Object 1, 1
Set object diffuse 1, rgb(255,0,0)
Sync On
Do
a = a + 1
If a => 255 Then a = 0
Set object diffuse 1, Rgb(a,0,0)
Sync
Loop
There is always one more imbecile than you counted on.