Unexpected Colour differences (MacBook Pro, OSX 10.11.6) I wrote a postscript file including a blue (0,0,255) RGB and a yellow (255,255,0) RGB shapes.

When viewed (in Preview) and according to the apple DigitalColor meter set to "native colours" mode, the colours are different:
     yellow becomes (252,255,54) 
     blue becomes (43,0,251)

To be sure the problem does not come from a bad postscript conversion, I used imagemagick to convert the postscript file into a raster image (png, tiff, gif,...). The so converted rasters show the same colour changes when viewed in Preview/Photo/finder. On the other hand, when imported in inkscape (i.e. X terminal), the colours of any of them are correctly displayed --- again, according to the DigitalColor meter.

How come? 

Should I change the setting of the color display to get it right? How?
Should I use another colour picker? Which one?
While I don't know the exact details, the Digital Color Meter that comes with macOS is affected by the color profile settings in System Preferences > Displays > Color. Changing the display's color profile from Color LCD to P3 to Adobe RGB to whatever causes a solid color to read differently under the same Digital Color Meter profile selection.

A photo editor with a dropper tool, specifically Photoshop, always selects the same color values, regardless of the display's color profile.