The colouring medium you use will largely determine what card stock you should be using to get the best results.
Use Whisper White Cardstock when colouring with Stampin' Blends or other alcohol markers. Whisper White Cardstock is a high quality card stock with an ultra-smooth surface, ideal for blending alcohol-based colour.
Use Watercolor Paper when watercolouring with water-based Classic Inks or Watercolor Pencils. Watercolor Paper is 300 gsm cold-pressed paper made from 100% cotton and is specifically designed for watercolouring. That means it safely absorbs the water and resists buckling when wet.
An alternative to Watercolor Paper is Shimmery White Cardstock. Unlike Watercolor Paper, this card stock will buckle when wet but it is robust and won't perish with water. The bonus is that your finished watercoloured image will have a delicate shimmer!