Hello,
Difficult to say from limited information I would suggest that you check that the plant is not too wet. The compost should be moist but not waterlogged. Drain the pot well and allow the plant to dry out a bit before rewatering.

Thank you, Melrt. I will try that. But what other info do you need?

Some reasons for chlorosis (yellowing of leaves while veins remain darker) in geranium -

Plants being overly wet (usually due to the lack of drainage).
The wrong potting soil.
The lack of quality light.
Advanced stages of nutrient deficiencies.
Heat-induced iron chlorosis during extreme, prolonged heat.

I think it must be overwatered. I've started to ease up on the watering. New growth was always green, drainage is good, I fertilize monthly and I use quality potting soil.
I'm going to put it in better light too. We'll see what happens. Thanks very much for your advise.

Dal....yellow leaves as you describe might be the result of your soil. If i read your location correctly, you are in New Mexico....and soil in south-western states is generally alkaline in nature. Heavy alkalinity can induce iron deficiency..."chlorosis"...which causes plants to go just the way you describe...in their leaves.

If you believ you have overwatered, that is easily fixed by letting the plant dry down more.....but if you feel the soil might be the problem, consider feeding your plant a fertilizer which will correct the chlorosis. Iron Chelate is usually how its described on the package. Normally, balanced fertilizers contain sufficient trace elements to help avoid such chlorosis.

A rusty nail, put into the soil beside the plant can often do that job...provides iron to the soil.

Chlorosis can also be the result of a Magnesium deficiency. It might be worthwhile spraying the foliage with a solution of Epsom Salts (Magnesium Sulphate). The solution should contain one teaspoonful of Epsom Salts dissolved in a gallon of water, spray the plants thoroughly but avoid "run off". Do not use this spray on tricolour zonals. The foliage will become noticeably darker green shortly after application.