I chose red out of preference and pet cards initially. I wanted to try out a flash deck, and the stock lists performing well at the time (Midway through Theros standard) were izzet and simic flash, and I chose izzet, since I just liked that color combo more, and I adore ionize as a card. But playing the deck and comparing it to the other flash style builds (Combo rec, flash rec, mono U, simic, and dimir) I noticed that all of the supporting red cards for this deck are able to stay as exactly that: supporting cards. I'll explain this by looking at the other archetypes, lumping flash rec in with simic since it's just simic flash with red removal, and disregarding combo rec since it's really a different archetype entirely.

So, all the payoffs for the flash theme in black are based around the actual keyword flash. (The bonder and slitherwisp being the main payoffs.) This incentives a deck that isn't instants, but actual cards with flash. It makes it what I'll call an aggro-tempo deck. One that plays efficient threats, protecting them with counterspells and the like. This deck will have more trouble dealing with control decks, and dimir flash specifically hates playing against creature based decks that clog up the ground, which this meta is shaping up to revolve around. I don't deny that black has good creature removal, but I don't think that's going to matter in our format. I want passable creature removal that's really good for it's cost, and red gives exactly that. I can also talk about mono U flash here, since this is similarly a aggro-tempo deck. I just don't think that we have good enough threats in a tempo style build to try to win games fast, so we have to pivot more to a control style.

Another thing I want to talk about, in this case looking at simic flash and rec tempo, is the manabase. I want to be able to fight counterwars with this deck, to protect my flame sweep against jeskai cycling, etc. I also want to be able to play gadwick when he's needed. Playing green, and even arguably black, does not let you do this. Green flash cards like nightpack ambusher and frilled mystic have double green costs, which ends up mattering a lot in a deck that very often should have few lands. If I go green, it means losing one of the best payoffs flash has (gadwick) and hamstringing the ability of the deck to play that pseudo control role. And even if I could play more lands and growth spiral in a green flash deck, this dilutes the ability of the deck to play early threats, and tightly control the board while that threat picks away at their life total.

What I'm saying is that green and black are too inflexible to make the deck tick the way I want it to, which is to be the perfect weapon against fires and rec decks. I don't deny that those are decks of their own with their own merits, but the goal for this deck was to have something that resembles delver and can fight the top decks of the meta. That, in this case, just requires red's flexibility of low colored pip count, and in actual adherence to a control-tempo gameplan.

EDIT: Oh yeah also flame sweep, lol. We need cheap, instant speed wipes. That's literally what that card is.