The old colors were Icon Grey (`#4d4d4d`, breeze) and an undocumented white color (`#f2f2f2`, breeze-dark). The new colors are Shade Black (`#232629`, breeze) and Cardboard Grey (`#eff0f1`, breeze-dark).
With this change, icons in GTK applications will be the same color as icons in Qt applications with the Breeze and Breeze Dark colorschemes.
I used `sed --follow-symlinks -i 's/4d4d4d/232629/' *.svg` for breeze and `sed --follow-symlinks -i 's/f2f2f2/eff0f1/' *.svg` for breeze-dark in each directory containing monochrome icons, excluding the color icons in `apps/16` and `apps/22`