Hey there Anthony, what did you end up doing with this space? Bit of a tricky space!

I would leave the render colour and just tile a trim around the base in a pattern and over the arch of the brick. But I also recalled the pizza oven at a pizza place where I grew up. They make great pizza, but the oven is so cool! You could do something like this!

Depending on your house style and how it would work with the 3 different measurements, you could look at doing a herringbone style pattern for example:

What natural light do you get in the room/what orientation? i.e. is it a 'cold' or 'warm' room? If it's a colder room, I'd go with a grey with orange/red undertones to warm it up a bit, or if it's a warm room you could get away with a cooler grey, maybe green undertones to compliment the maroon. Or you could even try a green rather than grey. The Aerobus is a blueish grey so will be cooler. As others have said you would probably want a lighter tone than this for the walls (unless you're purposely going for a dark dramatic mood). 'Miller Mood' is a couple of shades up in the Colour Atlas. Get sample pots and paint a section on each wall, or paint a white bit of card and stick to the wall to see how the light is at different times of day.