In your original shader, the diffuse value should be 1 (to give a full 0 to 1 range input to the Gain node) and the Gain node itself should have a high Gain setting (Value2) approaching 1.0. You had it as zero, which reduces contrast to nothing and produces a flat grey, instead of increasing contrast which is what we need. The shader worked to a degree, but almost by accident.

Miss Nancy used a Color_Math node (not necessary) instead of a Math node, but the gain value is in the right order.. near white.

Also the texture inversion seems over-complicated to me. I would use a single black and white brick node, and a Subtract node (1 - x) to invert it.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)