How to mapping number to set color

Hi. I am using a slider. I want to show Thumb color of slider as value.
When i slider thumb for 6500 to 12000 I want to get 3 values for RGB.
Example:
Red: 6500 to 12000 = R: 0 - 255
Green:6500 to 12000 = B: 50 - 255
Blue:6500 to 12000 = B: 100 - 255
My mean like Map on Arduino.

Have any one can help me?