Should I paint the apartment

I say no for two reasons: white looks "clean" and leaving it open for the tenant to paint lets them become creatively invested in the apartment, which is more satisfying to them.

We paint most of our units in white every 2-3 years depending on wear and tear. Darker colors will require primer or additional coats to get the white back later. It's cleaner, simpler, less expensive & more importantly less headaches.

YMMV

Most renters prefer to see clean, white walls. Remember, mustard yellow might be groovy for one person and and vomit-inducing for another. You'll drive yourself nuts trying to decide which color will be universally appealing to the unwashed masses 'cause no color is. And guaranteed someone will be bitching and moaning in to time that the Benjamin Moore "Azure Turtle Boob" you so carefully picked out and custom-mixed specially for them doesn't go with their new burgundy pleather Huffman Koos sectional. Stick with white.

Make sure it is an off-white. White is too stark. I only allow tenants to paint darker or non-traditional colors if they pay for a repaint when they leave.

A lot of people like "too stark" rather than "vaguely dingy". Brightest white makes rooms look larger, too, so prospective renter's first impression will have more "wow" power. That's money in the bank.

Depends on the lighting.

I decided to allow long term tenants paint an approved color if they want; the next tenant will just have to take the color scheme or repain themselves. I guess I stay with off-white in general

You could get a painting deposit from them, and require them to repaint in dingy dirty depressing not-quite-white at move-out if they want the money back. And a written agreement to that effect signed by both sides.

Keep it a light color if you go off-white. I like Bone White....

definately keep it to shades of off-white, no colors!