Why are the lemons on my lemon tree not turning yellow?

I saw a programme on tv (one of these b****y castaway type things) and on the prog. someone found a green fruited tree but wasn't sure if the fruit was edible. It turns out it was an orange tree and they don't turn orange in the tropics. What turns them orange is the cooler northern climate that we grow them in.
I wonder if this is the same for lemons (except, yellow rather than orange)?

I suppose it is a possibility, however, when I bought the tree it had 2 'yellow' lemons on it.

Try asking the same question on www.bonsaitreeforums.com as they have a citrus forum on there. Its a helpful and friendly site and someone will be able to help.

Sorry I can't be of more assistance.

THANKS FOR YOU HELP