What is the shade that is one shade darker than pearl?

I used to have pearl and switched to this one (no choice I think) and its the same to me.

Is the coverage good?

Yes! One coat is good for me, however you can add more if you need more coverage. This is not cakey. This first time I wore it I was not sure I liked it, now I love it!

The ad says taupe, but at check out it changes to beige...what is shipped?

You should be able to select the 'taupe' color on the right hand side. Otherwise it may be defaulting to beige. I checked on my side and it is working correctly. If it is still giving you trouble you might try closing our your browser and doing it again. Hope this helps!

Which foundation color is best for yellow undertones?

You can always return anything. That’s Amazons promise to you. How would you know if you don’t use it. If seller refuses return/refund, go to customer service at Amazon and they will handle it for you. I have no worries ever on Amazon that I will be wasting my money, that’s exactly why we buy almost everything here. Watch out for easy access containers like jars where they can change the product or just simply use and refill with a basic body lotion. Don’t ever use if anyone seal at all has been compromised. I am a customer and have no affiliation with amazon, nor did I get paid to write this! These are my own personal thoughts and opinions.