By Default, iMerchandise comes with 16 solid colors, one gold gradient and 4 flower and stripe patterns (fabrics)
Here's how you can add your custom Colors for your on the go inventory creation:
1. Go to Colors from the Main Menu
2. Tap on the + top right icon
3. Drag the 3 Red, Green and Blue sliders, until you get the color you want
4. Fill in the name and tap Done
Alternatively, you can do the exact same operation from the color window in the Product editing mode.