Is there a way to calculate how far apart two colors are?

I can't imagine being, say, Red/Green blind or the like. Must be a giant pain in the ass.

Mine is blue / green defective safe. When I first wanted to join the RAAF in Radio was told eyesight not good enough. Joined in medical

Part of medical duties was doing Ishihara test for colour blindness. Since I fail the test I had to learn the book by heart to know who was passing or failing

Time to get out 6 years later was retested and noted to be blue / green defective safe and allowed to change jobs to keep me in

I could read the colour bands on resistors and multi colour telecommunications cables

and dyslexic

Yes. Relating mainly to names

In textiles, objective measurements of color difference are as important as anything. The method for determining the difference is as I said. It's a distance magnitude in the three-dimensional color space between two points as calculated using the Pythagorean theorem.
I think you are being too literal with "far apart". If two people are "far apart" or even "miles apart" in their political views, is that a spacial distance?
If two people are "far apart" in their views then that is a metaphor that evokes an abstract space in which views have relative locations. If it did not do this, the phrase "far apart" would be meaningless.

If one doesn't want to suggest that things are being analyzed in an abstract volume, then one shouldn't invoke a spatial volume metaphor to describe it.
Probably yes. But may not be with these numbering system but may be with frequency of light of the given Colors.
Suppose I have a shade of blue with these values for the RGB attributes: 143 179 221. And suppose I have several other shades of blue, each with their RGB settings. I would like a way to calculate how close they are to the target shade. Is there an accepted way to do that? The RGB values for the other shades are in the table below.

R      G      B   Sum(Diff) Sum(Sqrs)
143    179    221       0      0
130    166    205     -42     24
167    188    214     +26     27
139    159    197     -48     31
172    186    245     +60     38

I tried two methods. One, Sum(Diff), was to simply add up the differences. That is a poor choice. If R & G are each off by +100 and B is off by -200, the Sum(Diff) will be zero.

My next method was to add up the square of the differences, Sum(Sqrs). This doesn't have the problem of Sum(Diff), but I'm not sure if it is a good metric.

Any suggestions?

Convert them to HSV and then use modulo operations.