This is assuming I do not have on my glasses. At night time certain fluorescent signs are quite sharp and crisp, but one particular color sign is always extremely blurry. Distance is not a factor.

What color is the sign, and why is it blurry?

I do glasses, not eyes, but I'll guess violet. Yellow light improves visual accuity and is in the middle of the spectrum. Red and violet are at the extremes of the visual spectrum, so it makes sense that it would be one of them. Ultra-Violet light is bad for eyes, in many ways, so I'll guess violet since it's closer to UV than red is.

I imagine that the lighting at night where you can see fluorescent signs is bluish, as in mercury vapor. The sign color that would have the greatest vibration against the blue would be...magenta. Just my guess. And I chose mercury vapor over sodium since the orange choices don't really have as bad a vibration problem, imho.

Why it vibrates (is blurry)

OK you both are sort of correct. The color of the sign is indigo, similar to this but more intense since it is back lit. It is almost the exact same color as those purple lights which indicate security phones on campuses.

Anonymous is bringing up a related phenomena of adjacent color effects. In this case though it is the single color sign against black night sky.

I'm thinking it has to do with the focal point of the wavelength of light being outside of the range my eye can focus it. I am also wondering if, being night, more rods are involved than cones and that may have something to do with it.

Many years ago, I noticed a problem seeing blue signs at night too. Figured it was for the same reasons. They are always very blurry when everything else is in pretty good focus.

I wonder if sign makers and marketing departments are aware of this? I bet it's fairly common.