What colors choices are you talking about? To the best of my knowledge only the imac G3, power mac G3, and ibook G3 gave you a real color choice. Cheap plastic macbooks came in white, or black for a small amount of time, before everything moved to the macbook pro metal chassis style that's used for everything now.
I wouldn't say having a color choice was historically common for them. That said cheap clip on lids \ bases are commonly available to accessorize all recent models.
You can have any color you want. Just do it yourself. Not willing to do that? Pay someone to do it for you.
Long and the short, this isn't really an Apple thing so much as it was a market trend/research thing. I went to a graduation party over the weekend. You know what color the graduation balloons were? Black. Not "regular" balloons from latex like they chose that color. But the helium filled shaped mylar balloons that say "Congrats Grad!" and all that nonsense were literally all black. They had 4-5 varieties/shapes... black. And the text was silver/gold with silver/gold confetti accents. Even just 5 years ago the trend of course was colorful.
As another marketing example case, you could look at the N64. They eventually made that console in every translucent color under the rainbow. And obviously they did so, once again because of the market demand for such a product. They haven't bothered with another home console since. Even the Switch is relegated to only two variations, and not a host of different colors.
The long and the short, the colors were due to making a product people wanted. People generally don't care about that market trend right now. Making you in the minority. So if you want a color, either do it yourself or pay someone to do it for you. If you're not willing to do either of those things, then it's probably not that important to you.