Before you start sharpening, mark one cutter with a felt tip pen. After two or three strokes of the file, check how much material has been removed. If the surface looks uniform, your filing action is correct.
If there are still patches of color in places, check that you are using the right file and make sure you are not holding the file too high or too low.
The depth gauge setting may be increased by up to 0.2 mm for cutting soft wood in mild weather conditions – no frost. The filing gauge for the next chain pitch up may be used for this purpose.
Never continue working with a blunt chain. A few strokes of the file at regular intervals quickly restores full sharpness.
Count the number of file strokes and use the same number on each cutter to obtain cutters of identical length.
If you find that the cutters in one row are shorter than those in the other row, you have used more pressure while filing the row concerned. Correct the length by with an additional one or two strokes of the file on the longer cutters.
Source - Stihl