Not sure what you mean by the OFF colour. Normally the LED graph has traffic light or red/green states. The levels for these states are set in th presentation tab. By default the item usually comes with two states, but you can add more if you need, or remove one so that you only have one state.
If the expression value falls outside the min/max range you define, the LED will show "unlit". Is that what you mean?
Select "Single Light" mode for this to work best.
Hope that helps