I have just this week completed eight 5x7 inch sketch cards(the biggest sketch card size I have ever worked on!) for Breygent Marketing and the Comic Con in San Diego next month. Boy, I wish I could go too. ;)
They've been fun to do, but I freely admit I'm not an accomplished comic artist.
I used watercolours, acrylic and pen for these cards.
I find the card stock tolerates watercolour, but tends to mute the colours somewhat, unless I use layers of colour. But then the surface breaks down a little if you use too much water.
Colour pencils struggle on this surface. The colours look very muted, though if you are after a softer look, then they're fine. I'm still finding ways to work with Breygent's card stock. It's pretty versatile.
Coloured ink pens are probably best if you are wanting to create vibrant coloured artwork, but I don't feel as confident using those yet- harder to correct mistakes!