Do you want to add the color to the actual, finished invitation or do you want to do it in your editting program?
For example, if you want to add a color after the invitation is printed, you could buy an ink pad in the color of your choice, and then glide the edges along the pad to give them a pop of color. Or you could run the edges around a glue pad, sprinkle the edges with embossing powder and then melt the powder (with a heat gun)... that would give it color and dimension (but is messy and pricey if you don't already have the tools for it).
Or if you want to add color to it before you print it, try adding a stroke to the layer you want colored. I use PSE, so here are my steps:
1) click the layer you want the edge colored
2) Edit --> Stroke (Outline) Selection
3) Choose your Stroke Width & Color, as well as where you want the stroke to be (outside, inside or centered on the edge).
Hope this helps!
