We construct redness-greenness (r,g) coordinates to fit the spectral locus into a triangle in the normalised CIE 2006 LMS plane. The reflection spectra for the Munsell patches for blackbody illuminants from 5000 to 6500 K appear as near circles in this space, suggesting that equal steps in (r,g) space may correspond to equal perceived colour contrasts within the gamut of reflective colours. We fit a matrix to convert from XYZ to LMS for reflective colours.