After a long, long time of toying around with various colour spaces 
(even including Munsell), I've come to the conclusion that none of them 
really fit the needs for creation of perceptually uniform gradients: 
Those which are ok for brightness gradients invariably seem to be 
problematic with gradients between different colours (sometimes for 
particular combos, sometimes in general), and vice versa.

So far the best interpolation method I've been able to come up with (and 
one which I think does give sufficiently good results for all gradients) 
is as follows:

- Interpolate colors in linear RGB color space

- Independently, interpolate brightness (Y) in non-linear space, e.g.

   Y_interpolated = [ ( Y_1 ^0.4 ) + ( Y_2 ^0.4 ) ] ^2.5

- In linear RGB color space, brightness-adjust interpolated color to 
match the interpolated brightness, i.e.

   (R',G',B') = (R,G,B) * ( Y_interpolated / Y(R,G,B) )

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