Here is a little description I wrote up on another board awhile back. I was confused myself at first, but after using them and refering to Patrick and other members I came up with this.
Propel Grey Finishing Pad: For the use of LSP's and the thinnest of products only, no cutting ability.
Propel Blue Polishing Pad: The difference between the blue and grey is that its built with a slightly stiffer foam for use of slightly thicker products such as an AIO. The pores are also slightly larger which gives a little more bite than the grey. The Propel Blue Pads are great for products that have cleaners in it or using for your first application of your LSP and glazes. It has extremely little cutting ability and has the least amount of cutting ability when comparing to all other Propel pads. In terms of comparison, the Megs Yellow pad is a step up from the Propel Blue Polishing, but below the Propel Green Pad.
Propel Green Light Cutting Pad: A more typical polish pad a lot like the Megs Yellow (8006), Lake Country White, or the Green Sonus DAS pads. It is the pad you would usually use as your "go to" pad. Most surface repairs will use this pad and can be used with products such as Meg's DACP & #80, 3M RC & MG, SSR 2.5 & 1. Can also be used with a rotary.
Propel Orange Versitile Pad: A different pad altogether, and takes some getting use to. Great for increasing cutting without the risk of much hazing as compared to that of the Lake Country Yellow. Products, speeds, and applied pressures can yield many favorable results. Great for use with a rotary.
Propel Yellow Medium Cutting Pad: Most aggresive Propel Pad available. Use when additional cutting action is desired. Usually used on heavier swirls or tasks requiring larger scale repairs.