Yellow or white variety tomatoes are very low in acid leaving room for a milder, sweeter flavor.
High acid red varieties have a good bite to them and are great for slicing thick and making sandwiches. Green varieties can vary in flavor from sour to sweet. They surprise me every time I try a different variety. Most green heirloom varieties are ripe when they are green.

Heirlooms are always picked when ripe and do not hold up for weeks at a time like commercial tomatoes. One thing to remember with heirlooms: buy 'em & eat 'em! They break down fast.