The controller process — perhaps after performing some computations using as input the multiplying factor sent to it by the device — sends a new factor down to the device over a dedicated channel.

This makes the device suspend itself and send the current value of the multiplying factor to the controller over a second channel.

You can also change brightness proportionally by multiplying each color channel by a value greater than 1 to increase brightness and a value less than 1 to reduce brightness.