A way to offset the colours of a multi-colour brush, either by palette order or gradient order.
Provide a spinner/slider to specify a integer colour index or gradient slot offset.
Could add a mouse wheel mode to change on the fly.
Could cover some use-cases of shade paint mode, but allow drawing multiple levels at once, and cleaner results due to replacing existing colours (with normal paint mode).
e.g. Use same three colour grass brush to draw various levels of shadowing by offsetting colours along one gradient.
Could be integrated into brush gradient mapping idea by simply adding the offset to the mapped gradient position.