Color Errors with Local File -- VLC vs. Firefox

[chris319]

I made an RGB .bmp file and encoded it to .mp4 at http://www.online-convert.com/. (I'm thinking this web site may be an on-line front end for ffmpeg.)

The .bmp image contains several patches of color of known value. Let's focus on the magenta patch in the image. The colors are:

.bmp file
R = 180
G = 16
B = 180

.mp4 - VLC
R = 191
G = 42
B = 183

.mp4 - Mozilla Firefox
R = 180
G = 17
B = 179

The colors are much closer in Firefox, +/- 1, which can be attributed to rounding error in the conversion process. This is a local file so there has been no resampling by YouTube, etc. The colors were measured with an eydropper program. It is the signal being sent to the monitor by the graphics card. The OS is Windows 10.

Note that the green value from VLC is 2.625 times the input value.

Why are the colors so much further off in VLC than Firefox?