I have almost all the G4 ponies except for international ones, so I could compare them to you. I display them by color, and I also have a good eye for color. I'm not sure how subtle of color varieties you want though.
Blue: Dewdrop Dazzle and Snow Catcher are almost exactly the same color, but Snow Catcher is slightly more on the green side and darker. It's very hard to tell though. Rainbow Dash is more sky blue. Unsure about Star Swirl.
Green: Lyra Heartstrings is the only currently released green pony.
Yellow: Fluttershy and Applebloom are the same yellow.
Orange: Scootaloo and Applejack are the same. Honeybuzz is extremely close, but with a tiny bit more yellow.
Pink: This one is hard because there are a LOT of pink ponies. Pinkie Pie, Cherry Pie, Lulu Luck, Twinkle Shine, and Ploomette are all the same pink. Sweetie Swirl is a tiny bit darker, and Starbeam Twinkle is slightly more brighter/saturated. Star Dreams (filly) is light pink and so is Celestia. Unsure about Skywishes, but she looks about the same as the majority of pink ponies. I'll let you know because I'm getting her in the mail soon.
Purple: Every purple pony is different (Cupcake, Cheerilee, Feathermay, Flitterheart, Twilight Sparkle, Lily Blossom, Luna, and Daisy Dreams). Not sure about Rainbow Flash, but she's coming in the mail soon so I'll let you know. Cupcake is actually purplish pink because she has a very faint amount of blue.
White: Plumsweet, Diamond Rose, Blossomforth, Rarity, and Sweetiebelle are all the same white.
Gray: Fashion Style Ditzy
Pony is the only gray pony.
Hope that helps!