Candles get darker when volume is high, and brighter when volume is low. They are red or green depending on whether the RSI is above or below a threshold value, or alternatively you can pick a more vanilla coloring based on current close vs last close or last open.
For personal use as I enjoy the aesthetics of it. The more solid RSI coloring helps highlight the brightness variations from changing volume and makes trends crystal clear.
Inspired by "Volume Based Coloured Bars" by KivancOzbilgic: