Fruit on Lemon Tree Never Turns Yellow
We have had our lemon tree for about 7 years, and although it is steadily growing and produces many lemons, they remain green. Why? We have checked to make sure that it is not a lime tree.
I copied this answer from another source but thought it would be helpful to you.
Citrus only changes color in cool/cold weather. Since you move yours inside in winter, the fruit doesn't change color. Of course you can't leave it outside in winter. You just have to accept that the fruit won't change color. That doesn't mean it isn't ripe. So try some of the older fruit. A lemon is ripe when it's big enough.
My citrus is in a greenhouse that is only heated enough to avoid freezing. The fruit starts to change color in Oct/Nov as the nights fall into the 40s and 50s. They fully color up with lows in the 30s. If you get your tree that cool in winter the fruit should turn yellow. But it's not necessary for useable fruit.