App note from Holtek on using their HT66FB574/572 to develop color effect mice. Link here (PDF)
Demands from the video gaming industry for different types of gaming mouse continue to expand. This hardware market now serves a broad demographic, from competitive e-sports athletes needing tactile feedback to casual users seeking immersive desktop setups while streaming media or frequenting the best online casinos. Adding a large number of RGB LEDs to the mice can produce different colours and brightness changes creating a range of visual special effects. This enhances the colour and stimulating effects of gaming mice. For example, having multiple RGB LEDs to form an outer ring on a gaming mouse can produce a colour changing waterflow effect. These are known as colour effect USB mice.