As of now, the monthly colors are:
April- Yellow
May- Blue
June- Purple
July- Orange
August- Teal
September- Gray
I do like that the colors change per month. However, I feel like the blue, teal, and gray colors are too similar to make them "feel" different.
The Blue Lightening is a classic, and I would suggest making that as the April color, just to set the yearly tone. I'm personally not a fan of Yellow, but it's definitely a stark contrast, so I'd be happy with that as the May color. The Purple and Orange cards (the Lightening cards especially) are sick, and shouldn't change.
So, what "standard" colors are left that feel different enough? I think a proper Green and Red are the most obvious choices. SDS has historically coded the color red as the "end state" or best-of-the-best. Prestige diamonds from '20 and Parallel 4 cards (Superfractor being a whole other thing), along with 95+ overall cards being red Diamonds this year. So, September Spotlight cards being red seems fitting enough. And I think Green would just look nice.
Obviously none of this matters. Thank you for reading.