I'm certain there's a way to do this, I'm just not too great with expressions yet--could someone help me with typing up an expression that would allow a Solid Layer's color always be the complimentary color of another layer (with a "fill" effect on it). I have tons of text layers that I'm precompositing, adding a fill effect to, and changing the color of by keyframing the fill effect over the course of the project. It would save me tons of time if I was able to link another fill effect on a solid layer, or just the solid layer's base color to always be the complimentary of the Precomposed Text layer's fill color. Hope this makes sense and thanks.
Adding the expression to Color property of your fill effect should do the trick:
target = thisComp.layer("Target"); //The layer you want to check (probably use a precomp for this)
colorpicker = thisComp.layer("Text"); //The anchor point of this layer determines where it checks
[1,1,1,1]-(target.sampleImage(target.fromWorld(colorpicker.position), [0.5,0.5], true, time)) //[1,1,1,1] is the RGBA value for white; by subtracting the result, we get the inverse.