When I'm writing a web page, I write text.
When I'm writing in a notebook, I sketch pictures, drawings, figures.
The most obvious analogue to sketching on the web is capturing input as a vector somehow. A mouse is a clunky way of drawing, but there is always Wacom and the iPad. Anyway, simple sketches are better than no sketches at all.
So, sketching. As with any technology, you should have a good grip on the requirements before you plunge into making something do anything.
On this page, I'd probably make a side-by-side comparison at the top of web pages and what they look like, and notebooks and what they look like. That would be a wire-frame diagram, effectively.
Then perhaps a cartoon of a frustrated user trying to use a mouse. I'm not a great artist, but doing sketches would also allow me to learn to draw a little better. I could make pages where I simply go over and over my own drawing techniques, trying to improve a particular design or idiom.
The idea of "requirements" is one of those annoying abstract things for which there is no obvious picture. So maybe I could draw some nice typographical thing for that: a handwritten balloon text of "requirements". Or, you know, something a bit more kosher for the intellect.
All quite standard stuff—and stuff that you'd find in any leading sketch application.
It's probably a good idea to limit the sketch app so that it's a bit like using a pen to sketch. Some of these sketch apps give you almost too much freedom. Some sensible defaults, some constraints, and away you go; that's a fine way to design an application.
As for colours, well that's one of the hard bits. Colours really have to be Munsell, don't they? And that means Munsell in JavaScript. Not only does it mean Munsell in JavaScript, but it means mapping Munsell to sRGB, and sRGB to Munsell. With perfect round-tripping. In JavaScript. Well, there's always Kickstarter I suppose...