You may find that your anti-perspirant/deodorant and clothing is more to blame for the darkness in your underarms. Mixing your own anti-perspirant/deodorant with plain, unflavored, Milk Of Magnesia, Aluminum Free Baking Soda, a drop of Tea Tree Oil, and your choice of essential oils for scent would aid in the skin healing up from its current state of irritation.
Additionally, wearing clothing that doesn't irritate the armpit is essential to the skin color returning to normal. Most clothing has a seam sawing at your armpit all day. A more body friendly design would be a gusseted pit, (as would the gusseted crotch, by the way) but very few designers use these.
Chuck Norris made a name for himself selling "activewear" in the late 80's and early 90's whose claim to fame was gusseting in the armpits and crotch to make it easy and comfortable to move, even if the clothing fit tightly, as was the fashion back then. Under Armour used to make their clothing in this fashion, but now is much less common in their line up.
These days, you could wear a base layer called a Rash Guard to have something soft and gusseted against your skin. The best,most reliable thing I have found for this purpose at the moment is: http://www.dickssportinggoods.com/produc...187736.11610236
I might also point out that I highly recommend these garments for wear during electrolysis work, as they are comfortable in any climate, soft to the touch, and have the aforementioned benefit of relieving stress and irritation in the armpit. When doing body work, they also add a soft barrier between your skin and the client's body. No scratchy or bulky fabrics.