> I'm wondering why, short of historical reason, does the
> interpolation of weighted textures(pigment) is performed in the rgb
> space.

Because it's easiest?

> Wouldn't it be possible to perform it in HSV or HSL space, even for
> out of rgb <0,0,0> / rgb <1,1,1> block ?
> And wouldn't the result be different and maybe more natural (or not ?)

Yes, IIRC some paint/drawing program has the option to do this, but I forget 
which one.

> Linear for SL/SV should be easy, but I'm still wondering about
> interpolation of opposite hue: what would be the right solution ?

Use something like Yuv space would be best, Y is the brightness and (u,v) is 
the colour.  But they key point is that equal distances between points in 
the uv plane are perceived as equal "differences" in colour by the human 
eye.  If you interpolate linearly in uv space you should get the most 
"natural" looking gradient without any sharp bits.  You can then just 
interpolate Y separately (maybe linearly or not, gamma is involved).

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