You're not mixing your compounds on the same wheel, right? Black on the sisal wheel for rough cut, green on colour stitch cotton wheel for first polish, then white on the loose fold cotton wheel for finishing polish. Remember to clean the work with Vienna Lime in between each grade.
Also, what size are your wheels, what are you spinning them on (hand drill or bench grinder) and at what speed? Your green compound definitely shouldn't be turning black! And do you occasionally clean the wheels? You can get a special metal comb but I find a stiff wire brush works just a well. Run the wheel and hold the steel brush against it to get rid of the dirt and expose some clean sisal/cotton
And AUTOSOL is fab too. I always use it after buffing to give that winning shine and keep it shiny for longer. Here's my power supply mounting plate after wheel polishing and a quick dab of AUTOSOL. A near mirror finish
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