I don't think it'll just apply to colour, but everything. Basically, any properties for anchors will be reported by getComputedStyle as if the link hasn't been visited (i.e. ignoring any a:visited rules, whether in your stylesheet or the browser's default).
Also, your initial premise is not correct. Bots will frequently visit the same page more than once (I can tell from my server logs), sometimes just a few seconds apart.
I think you'd be better off using some other method. For example, using the onmousemove and onkeydown events to detect things only a human could be doing. But this will require some thought, including accessibility-wise. For example, you don't want to alienate people using screen readers.