Ok, my bad, I should have re-read our lesson more carefully : we're actually supposed to do sRGB => XYZ > CIELAB (and later back).

And it looks like that you can either have an Euclidean vector (linear) space (linear sRGB, XYZ), or a perceptually uniform one (CIELAB), but not both !?

(I guess that I should have figured that out myself, sigh… this is why it isn't CIELAB that is used for monitor calibration, but CIELU'V' ? EDIT : Nope : "[CIELUV is] a simple-to-compute transformation of the 1931 CIE XYZ color space, but which attempted perceptual uniformity. It is extensively used for applications such as computer graphics which deal with colored lights. Although additive mixtures of different colored lights will fall on a line in CIELUV's uniform chromaticity diagram (dubbed the CIE 1976 UCS), such additive mixtures will not, contrary to popular belief, fall along a line in the CIELUV color space unless the mixtures are constant in lightness. ")

So you have to pick the best color space for the job, in the case of doing color averages that would be one of the linear ones (linear sRGB, XYZ), while if you are trying to design a perceptually uniform gradient for data visualization, you would better pick a perceptually uniform space (CIELAB, CIELUV) ?