You noticed that the RGB values used to represent ACI colors are not consistent across different computers or AutoCAD® installations.
AutoCAD® uses two different sets of RGB values for a given ACI color, depending on the current drawing background color.
If the background color in AutoCAD is set to black, the RGB color used to represent the ACI color will be slightly lighter. For all other background colors, a slightly darker RGB color is used.
Using two different RGB color sets increases the contrast of the drawing geometry, enhancing display quality.
Knowledge Base document: RGB values used to represent ACI colors vary
AutoLISP code to convert back and forth.