Sketch before colour

If you build a UI prototype that's too high-fidelity, you lock yourself in. Once you've got a colour palette, typography and a strict layout, you can't really focus on what's important, which is getting the information structure and hierarchy right first.

You want to know what the UX should be before you fill in the graphic design details.

It's like a painting. You don't start with colour, you start with a sketch.

That's what Ryan Singer teaches in all his material anyway.