Hmm, it's not really RED, it's a kind of a reddish-brown in the sprite and a paleish, vaguely red-hinting brown in the artwork.
Anyway though yes I probably did go a little too far into analysis, but it was to illustrate the possible points one could make. The biggest issue with mono-Grass, I think, is the lack of flavour precedent of the typings being reversed as Delta Nite and others have mentioned before. This is the main thing preventing me from being a more confident advocate of the typing, and there doesn't seem to be a way around it short of flipping Pyroak's types (unlikely).
As for Pichu/Darumaka/Psyduck... Pichu wouldn't fit as any other typing, really, though I suppose one might make a case for Normal. It can still use Thunderbolt and Thunder/Volt Tackle, though, and while I'm not positive on whether everyone agrees, I don't think people want Flamethrower or especially Fire Blast/Flare Blitz on Embirch (do correct me if I'm wrong), so it isn't as controlled with its implied element. Darumaka can't master the fire enough to be calm to wield it, yes, but it has enough fire to drive it nuts in the first place (and again wouldn't really fit another typing too well). Psyduck, no argument, save this tangent: it does have Psy- in the name when not Psychic-type, so Ember for Embirch can potentially have precedent.
As for Fire/Grass:
The Pyroak typing is as I said the most prevalent issue in my mind, and I could definitely make a case for Fire/Grass. Appearance-wise Embirch does not drastically differ from the other members of its line, not so much that it necessarily must be plain Grass. It looks far less practiced, and prepared, but that could be attributed to baby status or something along those lines perhaps (or really, just the fact it's a first-stage). It's got the cannons as bmb stresses, and the namesake to suggest that typing. Ultimately, I perceive Fire/Grass as the default that can't really go wrong if chosen- it makes sense, it's not breaking any canon. I present arguments for Grass in order to help even the playing field and because I prefer it in terms of raw appearance. Fire/Grass can be argued for in appearance too, though, especially with the current sprite.
It's the simplest way to go and I don't think it can really go wrong, I just think Embirch's looks fit plain-Grass more.
/potentially half-coherent sleep-deprived rant
My critiques on your art will be honest! I'm not mean about it- or at least I don't think I am- but I won't lie to you. As a result, I'll always hide-tag my critiques (don't be nervous because it's hidden!). My praise, however, is also honest, and I'll never give one without the other.