Drop all of your blond hair images into one file as separate layers.
Add a Hue/Saturation adjustment layer above them (Layer>New Adjustment Layer>Hue/Saturation). Move the Saturation slider all the way to zero to turn all of your hair layers grayscale.
Add a Levels adjustment layer above that (Layer>New Adjustment Layer>Levels). You'll use this later....
Add a Photo Filter Adjustment layer above that (Layer>New Adjustment Layer>Photo Filter) For this project, turn OFF "Preserve Luminescence" in the option panel. Choose the "Color" option and select your desired brown form the color picker. Slide the intensity slider all the way up to 100%.
Add a Hue/Saturation adjustment layer above that, but make sure to check the box to "Use Previous Layer as Clipping Mask"
Now, go back to that levels adjustment layer and play with the sliders until your tones look good.
Use the top Hue/Saturation adjustment layer and play with the Saturation until the colors look good.
Done.
If you want a different hair color, just add a second Photo Filter adjustment layer above or below the first one. Turn off the first one and play with the settings on the second one. For different hair colors, you may want to turn "Preserve Luminescence" ON.
