I just did some more testing on this. In the current case, you get the error version of the dialog ("Please close this window and try again."). When you click "more info", you get to see "improper usage of API: Error: invalid backgroundColor: floozy". So actually, I think that's pretty good behavior.

We could go with console.logging() a warning message about the backgroundColor and proceeding by ignoring the color passed, but I don't know that that's better. It seems like pretty much all RPs will only pick a color once, while they're setting up RP functionality, and for that case it would be better to fail as obnoxiously as possible. Once the RP is setup and in actual use, invalid background colors should be very rare.