if a character is prime, but doesn't have a green ring printed on the base, what is the ruling on playing with other primes? tomar-tu fell into this category at the WOL I was playing today, we let him do it because the rule book specifically references a green ring on the base

I would have ruled him as still being a prime. It should be clear from the card's green background that the character is a prime.

If Tomar-Tu was missing his ring, it sounds like it may have been a one off misprint. Every Tomar-Tu I've seen has had a ring. It's Galius Zed who is missing his ring and he was ruled to be a Prime.
http://www.hcrealms.com/forum/showthread.php?t=510705