rule  "Rule Dimmer to Color Item Brightness"
// TestDimmer121 is the item name of the dimmer value
// Mock_RGB_Color_x is the Item name of the color strip
// Openhab sends color data as Hue, Saturation, Brightness Data

when
	Item TestDimmer121 changed
then

	logInfo("rules","Dimmer to Color conversion Received " +TestDimmer121.state )
	var PercentType BrightnessfromDimmer= TestDimmer121.state
	var HSBType currentState = Mock_RGB_Color_x.state as HSBType
	var DecimalType CurrentHue = currentState.hue 
	var PercentType CurrentSaturation = currentState.saturation
	var PercentType CurrentBrightness = currentState.brightness

	logInfo("rules","HSBtype " + currentState) 
	Mock_RGB_Color_x.sendCommand(new HSBType(CurrentHue,CurrentSaturation,BrightnessfromDimmer))
	logInfo("rules","HSBtype NEW " +Mock_RGB_Color_x.state.toString) 
end

You did not post your item names, so you need to change to suit.
I do not have a HSB lamp node to physicaly test on, so i have only been checking the openhab logs.