I definitely agree that gender is multi-dimension. I'd never heard of a color solid, but I like the image it raises in my mind.
Personally, to explain gender and sex I use a series of spectrum starting with the gender binary (black and white), then using a series of colors arranged from light to dark to demonstrate varieties in sex (DNA, hormones, physical characteristics) and gender (identity, expression, socialization, interests). I describe examples of people at each line, and demonstrate how few people are 100% male/female on those spectrum. I emphasize the various intersex conditions when describing sex before moving into the discussion on gender to demonstrate how variations between male/female/other are natural. I then use an eighth spectrum, which I term dysphoria, which indicates how comfortable a person is with were they are on the sex/gender rainbow. Rather than ranging from male to female it ranges from none (completely comfortable with self both in terms of sex and gender) to very high (self-medication, strong need for immediate surgical intervention).