Robyn, have you tried to add the color map image to a math function node and setting that node to substract  before plugging the math node into the root node's displacement and/or bump node?
It makes a gray scale copy of the image map.

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OS: Windows 11 64-bit
Poser: Poser 11.3 ...... Units: inches or meters depends on mood
Bryce: Bryce Pro 7.1.074
Image Editing: Corel Paintshop Pro
Renderer: Superfly, Firefly

9/11/2001: Never forget...

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Today is the tomorrow you worried about yesterday