I can compare the colour of a surface to the colour of a standardised colour chip, which is as objective as, say, measuring something using a ruler.

Not quite. Colour is a three-dimensional subspace of the infinite-dimensional space of possible light spectra, but which subspace it is depends on the spectral sensitivities of your cone cells. OTOH I do think that the cone cells of the supermajority of all humans use the exact same molecules as photoreceptors, but I'm not quite sure of that.