Your problem may not be the fixative. Are you printing on clear film. If so, the decal will probably lose some color when you put it on a pen. Even if you put it on a white pen it will not look exactly the same as on paper. If you are putting it on anything other than white is isn't going to look the same. Like I said, even a white pen will change the decal because the reflective qualities of the blank (acrylic or wood) are just different than paper and paper is what it was on before you put if on the pen.
If you are printing on white film, you will still have the same problem, only not as bad and you can put it on any color pen and maintain your colors more or less
