I don't want to show all my cards here but out of anything on this forum, I would almost gamble that I know more about paint than anyone here, however I may not know shit about anything else haha... I have worked for a major paint company selling paint for over 10 years. I am not talking about the shit home depot sells either. If you are not going the Mylar rout or the Orca film I had suggested in one of my other posts use a standard interior or ext paint. I always hear people talking about using FLAT paint and that is fine I am sure. I would almost be willing to say any paint you buy, be it Flat, Matte, egshell/satin, semi-gloss, gloss, high gloss will all work about the same and you probably wouldn't see much difference in the plants performance from 1 sheen to the next but if you really wanna get picky about it and milk every ounce of growing potential hear me out.
I assume the reason people recommend flat is it defuses the light for a more even reflection. The higher the sheen the more focused the light can be at some points. That all being said, here is what I would recommend .....
If you are worried about mildew and shit like that get an exterior paint it has mildew preventatives in it that will help vs. the sometimes wet conditions of your grow area. I also recommend going with a Satin or Eggshell (same thing) finish instead of a Flat. My thought is Satin has a low sheen to it allowing it to reflect more light than a Flat but not so glossy as something above it to create hot spots. Also whats nice about Satin is its much more washable and resistant to marking up the wall. Now all that being said if mildew and things of that nature are not an issue or concern for you go with an interior paint.
The reason for this is usually interior paints have a brighter white base than an exterior. All companys have a standard tint base for pastel colors and another for mid tone and yet another for dark colors such as black. However there is often special colors that are even brighter than typical pastels, what could be called "Luminous" or High Reflective White.
Go to google and search
A87W00150 High Reflective White
Click the top result
This bright white base is only made in a hand full of products, I guess it could be called a special base. The paint listed above is going to be around the $40 per gallon mark due to its coverage and durability. But if you decide to go this way you can ask the person in the store. Whats the cheapest eggshell/satin High Reflective White or Luminous based paint you have? This is often a contractor series product that they maybe will cut you a break on, instead of paying for the top of the line product that they want normal retail customers to buy. Hope this helps.
Ahh something else I just thought of....That company sells sample quarts of Luminous White for under $6 in a satin finish that will cover an area of about 75sq feet. That will undoubtedly be cheaper than anything else they can provide.