Yeah, the colour wheel does infuse all of it, but when converting to DnD I think a lot of that wouldn't matter. Like, would it really matter if that barbarian is a red creature or that knight is a white creature? If you want to use the whole Protection from Colour or the Intimidate card ability then that might be important but personally, I'd rather keep the spirit of MtG but use what we have in 5e. If getting attacked by a horde of goblins, their shamans will probably be using red magic, elves are likely proficient in green magic, and that human wizard with numerous artefacts is probably proficient in blue magic. I'd even bring in some of the card abilities when doing conversions; creatures with haste might have a bonus on initiative, creatures with Protection from Red might have resistance to fire, etc.
You can fit the colour scheme in via location, red creatures frequent mountains, blue frequent islands or coastal regions, green live in forests, etc but in most occasions, I really don't think it would matter what colour something is in much the same way that alignment often doesn't matter in play (that is, it might come up sometimes, but in general it is just something written on the character sheet).