The colour axis is an object of the type TPaletteAxis. Here the code i use to format it nicely:

def ScaleZAxis(histname):
 	hist = ROOT.gDirectory.Get(histname)
 	ROOT.gPad.Update()
 	palette = hist.GetListOfFunctions().FindObject("palette")
 	palette.SetX1NDC(0.90)
 	palette.SetX2NDC(0.92)
 	palette.Draw()
 	ROOT.gPad.Update()