Having used Montana diamond grey previously I too looked for alternatives. Vallejo air range includes an insignia white, which looks to me to be whiter than your test, Adam, but a touch of their white grey gives a close match to the diamond grey of my AJA Aries. I will test a touch of bone white or aged white with insignia white which may be a warmer adjustment than white grey . At present my intention is to fix on a base colour/mix and follow Adam's advice to darken the base progressively for other shades.