It can be very confusing choosing ink colour combinations. Generally, you can get your cues from the catalogue - card examples and projects. Check out the colour combination used in the various Designer Series Paper. They are always listed at the bottom of the description.
I would begin with one ink colour when stamping the frosting and use the stamping off method to achieve varying shades of the one colour.
Alternatively, add a second ink either a lighter or darker shade and expirement with achieving different shades by 'stamping off'.
With this card I tried to match the frosting to the designer series paper. I used Soft Sky and Bermuda Bay. For the frosting base I stamped off once with Soft Sky, second layer used Soft Sky and finally,stamped off with Bermuda Bay.
Lastly, you can choose three different inks from the same colour family.
Here are some three colour combo's that I like -
1. Dapper Denim, Marina Mist, Soft Sky
2. Blushing Bride, Flirty Flamingo, Watermelon Wonder
3. Daffodil Delight, Peekaboo Peach, Watermelon Wonder
4. Crumb Cake, Soft Suede, Early Espresso