@matigo Ah, I understand that all modern browsers now support the 140 available color names. The color names should reference the same RGB values, so when you use "darkslategray" on one browser, you should get the same corresponding RGB value everywhere else.
Hex or RGB values obviously provide a far greater range of color choices, though—you're not limited to 140 colors.
As for CSS variables, I can definitely how they would offer consistency and make it easier to change things like key colors in a color scheme after the fact. They weren't available when I first learned how to code websites, and I'm only now just getting back into updating my knowledge of Web front end tech, so it's all new to me.