The DO-NOT-USE list is the discipline. Each rule traces to a specific failure mode that defeats the two-color premise.
✕A third accent color "for hover state" — use color inversion instead
✕Gradients between the two colors (defeats the discipline)
✕Full-color photography that hasn't been color-graded into the palette
✕Multiple accent fonts (one family carries everything)
✕Drop shadows in a third color
✕"Trusted by" logo walls in third-party color (use silhouettes in color-2)
✕Animated multi-step transitions (one signature motion, that's it)
✕Any deviation from the two named colors — no exceptions