An even easier way, if you want both sides of your layer to be solid colors, would be to just use the Fill effect, with this expression applied to the Color parameter:
frontColor = [0,0.5,0.5,1]; // turquoise
backColor = [0.5,0.5,0,1]; // dark yellow
if (toCompVec([0, 0, 1])[2] > 0 ) frontColor else backColor
In case you don’t know, in expressions colors are represented by arrays [r,g,b,a] wher r, g, and b are the amounts of red, green, and blue, expressed as numbers between 0 and 1. a is the alpha, just leave it set to 1.
Dan