Magnar is the only purple.

Agreed!

You can set your own color, and AI colors, just create custom factions and leaders with whatever colors you want.

You don't have to create NEW factions to change the color... the color is set in the xml files and I can change it without creating new factions.

In the CoreRaceConfigs.xml, you can set the default colors for each race, under Color tag.... not that hard to change yourself just need to use notepad (I recommend you back-up the file before you change the values there. But there you go... instant color change.

I actually did this in FE before they changed the colors of Kraxis and Magnar.  I remade the exact same factions of Magnar and Resoln so that they would use different colors and not clash with Altar and Pariden.  I stopped when I tried to recreate an exact copy of Gilden with new colors because custom sovereigns can't choose the Battle Hammer weapon like Lord Markin has.  I was pissed!  So now I want the freakin colors fixed the right way.  GET ON IT!

Let me see the colors of the rainbow are:

Red, Orange, Yellow, Green, Blue, Purple.  That's 6.  But Wildlands are using Green so we only have 5.  Let's add Brown, Black, and White.  That makes 8.  You might get away with Gray (but that will be really close to White) and you might get away with Pink (but that will be really close to Red).  If you want to have more than 10 opponents in a game, you're going to have duplication.  The better option and good UI strategy would dictate that the borders be clearly distinguishable.  When in doubt, think CIV V.