>I have not found a way to 'ligthen' the image using this rule.

Karl -

This might not be complicated enough, but set your display depth to 32 and try...

Display fill: Display boundingBox
		rule: Form blend
		fillColor: (Color white alpha: 0.1)

In a loop to 20 it does a reasonable fade to white.  Use "black" to darken, naturally.

	- Dan
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