Hi Kim.

In a more balanced quality of light, BM Cement Gray would have looked just as you hoped, a cool near neutral grey. But ALL near neutral paint colours from this approximate section of the color wheel, are prone to shifting lavender/purple-ish/plum even, if the lighting is less well balanced.

That doesn’t mean those colours are purple or have purple ‘undertones’ it’s simply an effect of light and context. To combat the problem, you might stay in the same region of the wheel but choose a more colourful option, to find something with enough chroma to stand up to the light.

Or if that cool light grey feel is something you are definitely seeking, then move either anti-clockwise, or even further clockwise, away from the colour which shifted purple. Sometimes even just a few degrees could be enough to find a colour which does not alter to the same degree. At 69.7° you can see that Cement Gray sits firmly in that ‘prone to shifting purple zone’. (But I need to stress this doesn’t happen in every situation, which is why it’s so important to sample in a space before committing.)

I’ve made a couple of other suggestions which might be worth considering. See how there Hue Angles differ on the wheel. I hope this helps a little.

🌈