Got another thought, though it's a quick and dirty, and would probably amount to no more than another temporary fix--only this time it would at least remedy the actual problem.  If, for some reason or by chance you can't loosen the works down there without a lot of trouble, what you could do is insert something down there--something small like part of a toothpick or a very small tab of metal.  I am not sure how often you use the color selector, but I could say that you could at least stay it in place with this quick and dirty method until you could effect a more permanent repair.  I'll try to find one of these machines and look in there so I can tell you a little better about what to do once you get inside.

It should be a fairly common machine.  I've seen them fairly often at flea markets and secondhand stores, and occasionally in antique stores.  You should run into one fairly soon.  When you come across one and can get it for a fair price, take it home and look inside both machines, but be careful not to buy one that has the same problem.  Rule of Thumb has it that if two or more machines have a common problem, chances are it's a design flaw.  I've seen this in cars I've owned.  Had two 1980s Chevrolet trucks that had a design flaw--they never wanted to start when I wanted them to.  Anyway, look at both machines, and if you got one with good spring tension in the color selector, you can either fix the one you presently have, or you can fix the one you bought.  Either way, you'll have a parts machine, and it's a good thing you have a fairly common typewriter.