I have my tomato plants in a 5 gallon bucket hanging up to mimic a Topsy Turvy, but one or two of the leaves on all plants are turning yellow. I keep them watered and fertilized with adequate drainage. I have 21 plants; only 3 or 4 are doing this.
Certified GKH Gardening Expert
The following article should be of some help to you: https://www.gardeningknowhow.com/edible/vegetables/tomato/yellow-tomato-leaves.htm