My battery icon has turned from green to yellow to tan. What does that mean?

I may have misled a little. Less than 20% is Red if you refuse the offer to go to low power mode.

If you accept low power mode then as Lawrence said it goes yellow.

You can also turn on low power mode at any time.

Also these days green means charging. If you were off charge and above 20% it should be black or white.

21%

20%

Green: 20% or more charged.

Yellow: Low power mode on

Red: Below 10% charged

It will switch to low power mode and turn yellow if the battery goes below 20%. You should not routinely let it go that low; charge it if it gets close to 20% if possible. No real harm if it goes below, so don’t be obsessive about it. And the reason not to let it get too low is more for you than the battery; you don’t want the phone to shut down due to low power when you need it. The battery will protect itself by shutting off, but that might not be the best thing if you are in the middle of an important call.

Settings, Battery, Battery health.

If that is greater than 80% AND the performance lets you use the phone a good 5 hours on screen per day then it is unlikely the battery needs replacing.

Check your Battery health