What Paige said about not coloring too close to the lines is correct--the color from the markers WILL bleed outside the lines a bit, especially when they are new and juicier. Something I saw on either the Spectrum Noir website or the Crafter's Companion website suggested using a piece of paper towel underneath your cardstock while you are coloring to help wick the color through the cardstock instead of it spreading.
As for the problem with the Memento ink, I'm not sure what is going on because that is the brand they specifically recommend for use with the SN markers. They do say that the correct cardstock really matters. I have been using my SN markers with the same
Georgia Pacific cardstock and not had much of a problem. Maybe try using a lighter touch when coloring?
You could also contact the wonderful folks at Crafter's Companion, who are very helpful. Here's the link to their website. At the top, on the right, you will find a link for "contact us." They might be able to help.
Good luck! And please report back if you find out anything helpful.