Use a color that is different from the typical, standard colors like white and beige. Try using brighter colors to make the gym look lively. Make sure to stay away from neon colors though --- anything too bright may be distracting or annoying. You could even try painting one wall one color and another wall on the other side a different color. Try using complementary colors for a look appealing to the eye.
Your Own Graffiti
Personalize your gym. If you or someone you know has an artistic side, try painting a logo on a wall with something that is unique to your gym. Paint your gym's name or your mascot on the wall, with the date your gym was established to make it a little bit of history.
Post your credentials. It's important for your clients to trust you and your certifications. It's one thing to say you have the credentials, but another to actually prove it. To verify your credentials, post any and all certifications you have on a wall that is easily visible. Make sure that you do this for anyone who is going to be training someone or leading classes.
Post candid photos. To build a bigger sense of community in your gym, take and post pictures of workouts or before-and-after pictures. Showing people's progress or showing the fun that people have at your gym, as well as what kinds of things potential customers can expect at your gym, can drive more business to your gym.
Mirrors and Lighting
Install mirrors on at least two walls of your gym to make it look larger. Mirrors are also good for people to check their form and for a trainer to help point out their errors or what they are doing correctly. You should also make sure to have proper lighting, neither too harsh nor too dim, for both safety and welcoming aesthetics.