Yellow leaves usually mean over or underwatering. Red leaves mean they were exposed to cold air. Geraniums don't like that.

Red leaves in geraniums can be caused by two deficiencies, as well. Chronic lack of phosphorus AND/OR magnesium is a fairly typical problem with container grown plants that are not fertilized properly.

If you are in the habit of fertilizing your plants with an all purpose soluble product (which will contain phosphorus), don't increase what you are doing. Phosphorus is not likely to be the problem in that case. You can add magnesium to your plants by adding a bit of Epsom salts to your water. 1 to 3 tablespoons per gallon of water will do the trick. You will only have to do that a couple of times over the growing season, unless you have a very fast draining planting medium and are in the habit of leaching your plants regularly.

This same solution can be sprayed on the leaves, as well. Foliar uptake is pretty fast.