Black =0, white = 1. To invert a map, use a math node to subtract it from 1 (don't multiply by -1 as that will just make the whole map less than zero). So: 1 - map = inverse of map. I'm not sure what you mean about compensating for positive numbers. The way I see it, all you really need to do here is control how strong the bump is on the skin and on the swimmers. To do that, set the texture strength back to 1, then adjust the bump amount until either the swimmers or the skink look right but the other one is too strong. Then go to bender 4 and and lower the brightness of the input colour for the part that has bump that's too strong. For example, if you get it so that the skin looks right but the swimmers look like the bump is about twice as strong as it should be, then you would set the colour of input 2 to be 50% grey. That effectively multiplies image 5 by one half.

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