It's been so long since I made a card I had to restrain myself with this one...so I plugged for a watercolour, which I haven't done in ages. I really love the effect, which I created using Tesco's cheapo 'Go Create' felt tip pens (for kids!), due to the absence of more grown-up pens :-) I scribbled on an acrylic block with each colour, and then painted using a paintbrush as if I was using watercolour paints. For extra definition and shading effects I used the felt tip pen directly on the damp paper.
The paper was just crappy copy paper covered in a couple of coats of gesso, an idea I was first introduced to by Mel at this blog post. It looks like seriously expensive embossed paper (see first photo in close up), and is really nice to work with for very little effort, just a little patience whilst it dries, but if you do a whole stack of this paper at once, you'll always have some to hand. I love how the watercolour 'bleeds' slightly down the grain of the gesso paint. You really don't have to be an artist to do this, in fact keeping between the lines doesn't look as good, hehe.