You don't say if the entire lawn is yellowish or if it just runs in streaks. However, if it is an established lawn, say two or three years old, and you mow it regularly, you may want to consider de-thatching the lawn. The thatch from the mower (unless you use a bagger) can compact near the roots and prevent moisture and fertilizer from reaching the root system. Some insects can cause yellowing, but usually their damage is circular or streaked in design. Finally, I would use an iron additive. These are available from your garden supply store and are easily mixed for spray or spread as pellets on the lawn....Best of Luck!