Cheers

Openhab and the MySensors binding use the HSTtype format (hue, saturation, brightness) for color information.
The binding automatically converts the HSBtype to the MySensors format of RRGGBB.
So you need to use the HSBType fromRGB(int R, int G, int B) function

items

Color   Mock_RGB_LED     "Mock RGB LED (HSB)     "  (test254) { channel = "mysensors:rgbLight:gatewayWIFI3:Mock_RGB_LED:rgb" } 
String  test_RGB_HexString "Test Color RGB hexS [0x%s]" (test254) 
Number  test_RGB_Red	" Test Colour Red    [%s]"  (test254)
Number  test_RGB_Green  " Test Colour Green  [%s]"  (test254)
Number  test_RGB_Blue   " Test Colour Blue   [%s]"  (test254)

Rules

val String filename = "rgb-light.rules"

rule "System startup"
        when
                System started
        then
/*Initial value of colors*/
    if (test_RGB_Red.state == NULL) test_RGB_Red.postUpdate(0)
	if (test_RGB_Green.state == NULL) test_RGB_Green.postUpdate(0)
    if (test_RGB_Blue.state == NULL) test_RGB_Blue.postUpdate(0)
end
rule "Test Colour Channels RGB to Mysensors"
when
	Item test_RGB_Red   changed or
	Item test_RGB_Green changed or
	Item test_RGB_Blue  changed
then
    val int r = (test_RGB_Red.state as Number).intValue
    val int g = (test_RGB_Green.state as Number).intValue
    val int b = (test_RGB_Blue.state as Number).intValue
    logInfo(filename, "Input Conversion: r:" + r + " g:" + g + " b:" + b)
   var HSBType hsb = HSBType.fromRGB(r, g, b)
    logInfo(filename, "Input Conversion: hsb" + hsb)	
    Mock_RGB_LED.sendCommand(hsb)
end


rule "RGB Hex -> HSB"
when
    Item test_RGB_HexString received update
then
    var rgb = test_RGB_HexString.state.toString

    var r = Integer::parseInt(rgb.substring(0, 2), 16)
    var g = Integer::parseInt(rgb.substring(2, 4), 16)
    var b = Integer::parseInt(rgb.substring(4, 6), 16)

    logInfo(filename, "Input Conversion: r" + r + " g" + g + " b" + b)
    var HSBType hsb = HSBType.fromRGB(r, g, b)
    logInfo(filename, "Input Conversion: hsb" + hsb)
    
    Mock_RGB_LED.sendCommand(hsb)	
end

/* I find this most useful for testing the node the Node */
rule "Test Colour Channels RGB to Mysensors"
when
    Item test_RGB_Red   changed or
    Item test_RGB_Green changed or
    Item test_RGB_Blue  changed
then
    var int r = (test_RGB_Red.state as Number).intValue
    var int g = (test_RGB_Green.state as Number).intValue
    var int b = (test_RGB_Blue.state as Number).intValue
    logInfo(filename, "Input Conversion: r:" + r + " g:" + g + " b:" + b)
    var HSBType hsb = HSBType.fromRGB(r, g, b)
    logInfo(filename, "Input Conversion: hsb" + hsb)	
    Mock_RGB_LED.sendCommand(hsb)
end