https://www.inaturalist.org/observations/12531286
https://www.inaturalist.org/observations/98612418
https://www.inaturalist.org/observations/100174099
All the same species yet vary greatly in body coloration. Is there a known reason for why this is?
There are many ways to answer this question. Selective, genetic, developmental, etc. answers can all simultaneously be correct. One place to start looking for an answer that makes sense to you is:
https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=intraspecific+color+variation+in+spiders&btnG=
You can see a similar variation in say - a stand of ericas. All the same species but colour varying from dark to light pink, an odd cherry red one, maybe a white one.
One way to get to know how much variation can be found in different species, is to look at things like species with names like versicolor
https://www.inaturalist.org/search?q=versicolor
Many variable coloured species will have similar names.
Of course you have species like “Haematopus unicolor” which conflicts with its common name “Variable oystercatcher” though when it was named it was likely named after a particular morph.