Warren should be purple or red while Biden should be blue.
Purple on the Atlas is often used to represent a minor candidate's percentage (at least in the primaries:
see literally any primary result page).
I personally am partial to the following color scheme:
Green for Sanders: I think the most obvious color match; Sanders was green in 2016 and is running again. He's still a strong candidate and it even matches with the idea of being more third-party.
Red for Harris: Harris is, to me, the most analogous to Clinton (the candidate who has been red in the past two Democratic cycles). She's the more moderate, female alternative to the main candidates in the race.
Blue for Biden: While Biden was represented as orange in 2008, the last time he ran for president, he dropped out before the Iowa caucus and thus only won votes because he was already on the ballot in a couple of areas. For pro-blue reasons, blue is the Atlas color for Republicans and, while I don't think Biden is a Republican by any stretch, he is pretty clearly the most moderate of the candidates, at least those in the top-tier.
Orange for Buttigieg: Orange is not really all that frequent a color on the Altas. However, it was notability used for Trump in 2016 on the Republican side. While in no way a Trump of the Democratic primary, Buttigieg does represent an insurgent campaign that rose out of nowhere to become, for a time, a top-three figure.
Yellow for O'Rourke: Again, a not super-common Atlas color. And, again, most recently prominently used on the Republican side. O'Rourke, like Cruz, is from Texas. ¯\_(ツ)_/¯
Tan for Warren: Running out of colors, I don't think tan has really ever been used in past races. However, with so many candidates, I think Warren is the figure with the least analogues to prior races. As such, Warren gets to have the least analogues color.
Noticeably absent from this list is Yang. This, and Warren being given the color tan (among other details), can probably be chalked up to the fact that I made my color decisions for
my own polling project back in late March when Buttigieg was just on the rise, Warren was flat, and Biden hadn't even entered the race. However, unless there are more substantial changes, I think that this color scheme still works fairly well. And I, at least, buy my arguments for why all the colors should be the way I see them.
As for the other question, Arkansas seems to me like a Biden state: Southern, Moderate, and with a sizable black population.