Is it normal or expected that sometimes after a session of fire kasina practice that a particular color will vibrate outside of practice?

For me it was sidewalk grey/brown. The color would vibrate very quickly, in a rather unpleasant way, as if the sensitivity levels were turned up for me.

Also, is there any recommended action in response to this?

Thanks in advance!

Meditation in general will tend towards increasing the prevalence of visual snow.  I haven't done much fire kasina, but when I've done other visual-field focused vipassanna it definitely increased the amount of visual snow I experienced as well as made me very aware of lights flickering. 

One way to think about is that you're more aware of the 3 Characteristics of visual phenomena now. You see that the visual sensations are rapidly changing, and that's impermanence.  You noticed that it seems to happen in a "rather unpleasant way", and that's suffering.  If you do enough of it, at some point you'll notice that all the visual phenomena are being processed independently of you "looking at" them with muscle tension in your head, and that's not-self.

If it really bothers you, stopping the practice and changing to something else will probably help.  I used to get a lot of visual snow, but I don't get that much now since I discontinued visual field work a year ago.  But if you're making progress with insight, there'll occasionally be something disquieting happening almost no matter what.

Thank you! This was very helpful!

My general instinct is to keep working and not let stray phenomena distract me, but it's good to check in and see if there's anything practical that I should be addressing.

I spent 15 days on fire kasina last year.  At one point in the middle, i was walking around between sessions, walking towards a shady campground area where there were tents under the trees.  I noticed a green blob bobbing around the area of some tents.  I looked a little more as I got closer and I saw that the green blob was actually someones head, not too weird....  I got closer and saw that it was a person that had *white hair* but for some reason my visual system had just been calling it out to me as a green blob!  OK, that's not vibrations but just a typical weird meditation/kasina story.

Anatomically, I think it's just that sensory data gets filtered through 'sub-minds' ('The Mind Illuminated' by John Yates), the sub-mind output is what 'we' actually are generally aware of and our awareness is always changing from one sub-mind output to another. In contemplative practice we task the sub-minds with handling the sensory data differently and they respond in unpredictable ways so we end up going through Times of Weirdness. My interpretation of one of the common weirdities is that a gateway (submind) to a given set of sensory data can alternate it's output rapidly, such that we see/feel/hear/taste/etc vibrations in that area. This might just be that our awareness flickers back from something else to the given sub-mind output. Flickering happens to me practically any time I keep attention on sensations in a certain way, I just automatically start feeling flickering light/feelings in the area of eyes/vision. It makes sense to me that this might happen with just certain aspects of vision. Some months after I started vipassana practice I noticed, when reading a book, that the darker area in the fold between the left and right side of the open book markedly flickered. I've heard that I'm synthesthetic (not uncommon), so it might be that my 'symptoms' are not applicable to you.

Vipassana-wise, all these details might serve to relieve fear and promote additional exploration, but as JP said it's the recognition of their True Reality (not-self, unsatisfactory, and impermanent) that points us in the Right Direction.

JP's recomendation to just take a break if gets too weird sounds good to me too!