The animal Embirch is based on is a Salamander, which are commonly found as red:
Meanwhile, chickens are brown or beige, and chicks are yellow or pale yellow, so the red is used to indicate fire typing. I feel Vulpix is a fire-type because there is no other alternative; if Ninetales is a mono-fire, than Vulpix should be as well. With this situation, you have the alternative of choosing Mono-Grass type, as there is that option.
As Salamanders are already red, the colour of Embirch's body isn't to show fire-typing, but to keep true to the Salamander aspect.
Also, DHR-107, can I suggest to take off Mono-Fire option.
This is because I feel/know that this option isn't going to win in the poll, so we have a larger amount of people voting for the two prominent and viable options, Fire/Grass, and Grass.
@Deck Night: I feel Embirch is different. It's not just that it can't store that much flames, it's that it looks like it will self-combust if ever it uses those cannons.
Anyway: Mono Grass is the way to go!