I'm new to all this, but it seems that food coloring would break down in sunlight quicker than other dyes. I normaly lose things far faster than that, but for those who remember where they put things, that may matter. Just assumptions though.[]
by using a topcoat of CA over the food colouring will prevent deterioration of colour. The same would apply(pun intended) when colouring with a highlighter marker. CA does the trick everytime.
I use aniline dye mixed half strength; apply to pen just before I put on the CA finish. I use yellow for just a hint of color. Not a fan of the dark colored cob pens that some are making. It just a personal preference thing, but the light yellow seems to add just the right punch to the pen.