def setTileColour(node, red, green, blue):
    '''convert normalised rgb values to a hex value and use the result to cange node's tile_color'''

    hexColour = int('%02x%02x%02x%02x' % (red*255,green*255,blue*255,1),16)
    node["tile_color"].setValue(hexColour)

def tileColourCB():
    '''callback script that parses a user knob's va;ue wen it's changed and calls setTileColour based on the new knob value'''

    node = nuke.thisNode()
    knob = nuke.thisKnob()
    if not knob.name() == 'myColourKnob':
        # if any knob other than the custom knob is changed, don't do anything
        return
    # store colour values for each label used in the custom knob
    colourDict = {'red':(1,0,0), 'green':(0,1,0), 'blue':(0,0,1)}
    # since the custom knob is a cascading pulldown choice knob, split the label and only take the last item as the requested colour label
    colourString = knob.value().split('/')[-1]
    # use the colour label to look up the colour values from the dictionary
    colourValues = colourDict[colourString]
    # assign the colour to the node
    setTileColour(node, *colourValues)

nuke.addKnobChanged(tileColourCB, nodeClass='NoOp')