Operating system: Windows 8
Graphics card: RX580
Broken: 2.80-a1ad71304c2d-win64 (latest)
Short description of error
In object mode, with solid display, flat lighting, and vertex colors my objects have the right colors in the display (or at least very close to the right colors). When I enter vertex paint mode the selected object gets darker.
Exact steps for others to reproduce the error
Open a new general file. Make sure you are in solid display mode with flat lighting and vertex colors (from the drop down next to the display modes in the top right of the 3D view). Enter vertex paint mode. Right click to choose a color (I used 0.2, 0.8, 0.4 RGB). Shift + K to fill the cube with that color. Switch between vertex paint and object mode to view the difference.
You can also hover over the cube and then press s to sample the color, then shift k to fill the cube again and the color will change. After doing this a few times my cube turned black.