Welcome to the forum, Boshen.
I was playing with some VRML exports yesterday and I noticed colors were being substituted. Creo does look at color difference to see if they should be combined. This is a very old feature in the render engine when number of colors had memory implications. It seems to limit the number of colors the export formats will take on.
I don't know how .obj files work. If they are anything like VRML, you can manually change the color with a simple text editor. The only reason I know this is that I have done several years of VRML manipulation for efficient files in virtual worlds. And I can tell you that the VRML export was not even close to the color defined in Creo although the appropriate parameters are exported.
If the .obj files are simple text, and you have access to the standard for these files, you may be able to do a lot more with them as well, including applying textures, smoothing, and other useful features. Do not expect Creo to make use of the power that each standard allows.