Levels

When applying a “Levels adjustment,” you can set black and white points in order to decrease color tints, but where are the darkest and brightest spots in the image? Go to New Adjustment Layer → Threshold to find those areas. This function is available under the “Layer” menu.
Move the slider so far to the right that only a few white spots remain in the document. Use the “Color Sampler tool” and set down a point there. Move the slider to the left until only a few black spots remain, and set a second point down there.
One could also find a neutral gray in the image by using a “Threshold adjustment layer.” Add a new blank layer between the original image and the threshold adjustment layer, and fill this layer with 50% gray. Go to Edit → Fill or press Shift + F5, then select “50% Gray” under “Contents” and click “OK.”

                                         Here is the threshold adjustment layer at work.
 
Change the blending mode of this layer to “Difference.” Select the “Threshold adjustment layer” again and move the slider all the way to the left. Slowly move the slider back to the right until black dots start to appear. These are the neutral gray areas in the image (if neutral grays are present). Add a “Color Sampler spot.”
Now delete both the threshold adjustment layer and the 50% gray layer. Create a new adjustment layer, “Levels.” Use the first Eyedropper tool to click on the darkest area, then use the third Eyedropper on the brightest area.

                                                   Here’s a before-and-after comparison.
 
Now you can use the gray Eyedropper tool on the third Color Sampler spot. The color tint will be decreased. Color Sampler spots can be deleted by dragging them off the canvas with the Color Sampler tool.