OK, there is something really weird going on. This morning, I am positive that when I saved a document with colors set, closed it and opened it back, the colors were preserved. The document was a Body with an additive cube.
But I'm trying it again, and now it doesn't work. Furthermore, I tried a second time with a new file, and as soon as I set colors on the additive cube, it suddenly hid itself and there is no way to turn its visibility on again.
Third try: create Body with additive box, set color of top face, then save document (previously I was saving the document before setting colors). I close it and open it back: color is preserved.
Edit: 4th try, new color for front face of additive box: color is lost at reopen.
2nd edit: two more tries, both successful. It's a roll of the dice...
OS: Ubuntu 18.04.2 LTS
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.15897 (Git)
Build type: Release
Python version: 3.6.7
Qt version: 5.9.5
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: French/Canada (fr_CA)