John,
"Yellow" Betelgeuse is probably just perception.. BTW, some people are partly colour blind ( protanomaly, see
here) , and what they may see as "yellow" could be more red to the rest.. In science, we want confirmation of claims from couple of sources... if there is only one source, it could be considered as not reliable.
BTW, couple of thousand of years is very short time in evolution of stars... that applies even to short-lived supergiants, like Betelgeuse),
So far Betelgeuse seems to be at the end of it's red supergiant phase.. which may last hundreds of thousand of years - so it is very unlikely it was yellow giant 2 thousands years ago. But, if this is actually the case (some people are currently working on such models), that means there will be no fireworks any time soon :-)
And, stars do not just dissapear.. so those 50 missing that you are mentioning are most likely still there (or someone just wrote a fake news somewhere).
Or there is much more to it than just dissaperance.
If a star explodes as supernova, it will be much fainter (in visible light that is) that progenitor once the fireworks is over..