How about using the Slider object? That control provides a continuous sequence of values . If you need a corresponding color intensity display, you could use the Color object and either use discrete colors of lighter to stronger value or use the RGB object.
For how to control the Arduino/Bluetooth,just google within the forum using the Search for topics box at the top of this screen and to the right of the colorful Google with search terms lik arduino or bluetooth.
Regards,
Steve