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- South Africa
If it's a whole image then the simplest way is to convert the image to Grayscale, and then back to RGB mode. i.e Image>Mode>Grayscale and then same but RGB.Where do I find a b/w neutral filter in Photoshop? (menu) (please)
Alternately you can use a Black & White Adjustment, which offers more choices and adjustments. Adjustments can of course be applied to individual layers. Sometimes that's important, most of the time a simple back and forth does the trick.