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?