Striping in the grass is a common result of uneven fertilizer application. It's possible that the spreader wasn't working properly, or that some areas of overlap got a double dose of fertilizer. You have several options. You could apply more fertilizer to the light green areas only, but it's difficult to get an exact match to the stripes. You could wait for the color difference to fade; it will even out eventually.
To avoid this in the future, it's a good idea to fertilize at half the recommended rate, but go over the area twice at right angles to each other. Also, broadcast spreaders are less subject to striping than are drop spreaders.