It all depends on your style and preferences. You can go for vibrant and uplifting colors like a burnt orange, something sober and soothing like turquoise. or neutral earth tones. If the exercise room is in a typical basement, without a lot of daylight coming in, keep in mind that colors will look at least two shades darker. Compensate for that by choosing lighter shades of the chosen color.
Another great idea to create accent walls in basements is to use colorful wall decals over light neutral backgrounds.

"We dug a hole in the ground 12 inches down, poured a new slab at the lower level and poured little walls with a curb surround so the hole wouldn't fill with water if the basement got wet." Water seeks the lowest level and will seep in around the edges. This new deeper part will be the first area to take on water if it gets under the surrounding slab. Hopefully drain tiles were installed around the perimeter of the new slab to direct water to the drain to prevent this from happening.