Some tomato varieties are yellow and other colours,  and the colour has no bearing on ripeness.  The yellow are low-acid and have little flavour.  The only way to get any flavor in a tomato is to grow your own or purchase from a farmer.   Grocery store tomatoes taste just like plastic .   The taste can be improved by slow roasting (then stored in your freezer) but nothing comes close to the taste of a 'real' tomato.  Ironic isn't it, that after breeding all the flavor out in order to substitute colour and shape, the scientists are now trying to reverse the process.