Why do my geranium leaves turn yellow?
I have my plants on the window sill with light but not direct sun.
The first thing to check is soil moisture. Potted plants often show yellow leaves when the soil is too dry -- water is not getting to the leaves -- or too wet -- roots are damaged so they can't transport water to the leaves. When you water, pour on enough that you get a runoff from the pot. You don't need to pour it off if it's 1/2" or less. Before you water again, test the soil all the way to the bottom of the pot for moisture. You can use a pencil, dowel, or kebob skewer as if testing a cake; or use a moisture meter. The soil should be very slightly damp.
After you decide if soil moisture is the problem, think about light, soil, and mineral levels. This article has more information: https://www.gardeningknowhow.com/ornamental/flowers/geranium/geranium-yellow-leaves.htm