I did try something called Gen X for a time, it's works as you described. It tuned my hair a shiney grey that looked obviously colored.
The problem with me, and other male seniors, is the rest of our body hair turns white and even our skin looks bleached out, so when you color your hair it's kind of obvious.
I'm waiting for a pill you can take that colors all the hair on your body the same color, from the inside out, maybe that will be more natural looking.
As for thinning hair I don't plan on trying to fight or hide that, I just keep cutting it shorter.