You can use an Rgb bundle to model colors in hardware. This Rgb bundle take as parameter an RgbConfig classes which specify the number of bits for each channels :

case class RgbConfig(rWidth : Int,gWidth : Int,bWidth : Int){
  def getWidth = rWidth + gWidth + bWidth

case class Rgb(c: RgbConfig) extends Bundle{
  val r = UInt(c.rWidth bits)
  val g = UInt(c.gWidth bits)
  val b = UInt(c.bWidth bits)

Those classes could be used as following :

val config = RgbConfig(5,6,5)
val color = Rgb(config)
color.r := 31