New adjustment layers are set to Normal blend mode by default. If this is left then any alterations made to the exposure of an image will also affect colours
Whenever you make changes to the lighting of a photo using an adjustment layer, such as Levels, change the blend mode to Luminosity.
Luminosity makes sure that only the luminance (or intensity of light) is affected, and colour of the image is left alone.
The same goes for adjustment layers used to alter colour, such as Photo Filter. Change its blend mode to Color and your image will look slightly different, if not better!