Now when I'm doing the afterimage meditation I'm able to keep the image in sight until I reach level 2 on the closed-eye hallucination scale described in Wikipedia. It takes me 3-5 minutes of focusing on the afterimage, and focusing on my breath as needed to still any internal dialogue, to reach this stage.

Closed-eye hallucination - Wikipedia, the free encyclopedia

I find that reaching level 2 here with the afterimage still in sight is helpful, because the afterimage works as a template where for me to fill in with the colors that are flashing by my closed eyes. The colors tend to move around a lot and the afterimage helps me have somewhere to move them to hold them still.