I have been using WD-40 on my guns since about 1970 exclusively. I use it for cleaning and lubricating, and I clean my guns after every range session. Guns I have had since that time still show the the original blueing without fading or discoleration.
One must take care to reduce or eliminate the WD-40 inside carry guns as it can penetrate the primers. I have a small air compressor to blow unwanted WD-40 off and from the inside of the firearm. I also use WD-40 for long term storage and have never had any problem with rusting on any gun.
For lubrication of the slides on my semi automatics and on the inside of my revolvers I use Break Free CLP.
Just thought you would like to know.