I am drawing a map in Inkscape

[kiriona]

That doesn't look like a height map to me, it looks like a light map. If you have an actual height map of this terrain (where the value of a pixel indicates its elevation), then you can do your hypsometry programmatically instead of having to draw it by hand.

Ah. No, this is not a height map of that type. (I'd be curious to learn how to make and use them, though...) It's just a greyscale shaded relief image superimposed over the map at 35% opacity. (As an aside I figured out why it was taking me so long—each bit would take about a minute to render—and it's because linked images slow Inkscape down a lot. With the shaded relief removed [I'll put it back later] it works a lot faster.)

Colour six of eleven now xd

@ Michael Stenmark - It's for teaching myself how to use Inkscape, mostly. I'm half-heartedly inventing a country at the moment—a vaguely Anglo-Celtic one to judge by the river names—but I doubt I'll do anything much with it right now.