FYI, the phenomenon you describe is an afterimage and happens to everybody (http://en.wikipedia.org/wiki/Afterimage). You are correct that the visual cortex is involved in both phenomena but they are quite different. An after-image is still an "upstream" image on your retina, whereas a mental image is not. Whether the former can be used to increase facility with the latter I do not know. I haven't seen scientific evidence that imagery can be trained at all.
Despite all that it doesn't sound like a terrible place to start.
Despite all that it doesn't sound like a terrible place to start.