I can understand your concern regarding the colour red. It is a strong colour that will attract attention, perhaps more so because it will be surrounded by lots of green.
On the other hand, in your composition there are two lines (one formed by the running roman + horses + chariot; the other by the menhir, crouching Celt and dead Roman) that point towards the Boudica figure. This technique is often used in Renaissance painting to direct the eye of the viewer towards the real subject of the painting.
The Open University had a post on LinkedIn that explained this in a manner, and used a picture (can't remember which one at the moment) of a large number of Dutch soldiers sitting at a table. The central figure was seated with his back to the viewer, but looked towards the viewer over his shoulder - so I thought this was the subject of interest, but as was explained there were two subtle lines pointing to the figure on the far left who ended up being the most important one of the lot.
Personally, I find this the most challenging aspectc of art. There is also the 'rule of thirds' to take into account. but when I look at the overall picture, my eye is naturally drawn to the chariot as it is. That may change with adding the read colour, but then again, it may not.
