While pre-A2k4 allowed only 256 colors, you could modify the ACI index to virtually any color ... but you could only have 256 of them...This is not done in AutoCAD but rather in the video card settings if your card allows such reassignment of colors. The problem is, that while you can see the nice shade of pastel blue on your screen, it will be interpreted by your plotter as the ACI color you replaced ... and it will show up as the original color on every other system unless they too had the same display corrections....
In A2k4 Autodesk included the ability to use a true color RGB to accomplish this.. essentially you specify the reg, blue and green depth and it is applied to the object.