FWIW, I finished the inside of my growdrobe with flat white, and it worked great. Reasonably cheap, available at the local building store. What's not to like?
You might gain a few percent reflectance with higher tech options, but I doubt that a few percent of reflectance means a lot compared to the light the plants get direct, especially in a larger space where walls are further away from most plants. The plants in my first grow in the growdrobe suffered from too much light, not too little.