Four Ways to Convert Colour to Monochrome

Image - Mode - Grayscale (converts RGB to Grayscale)

Image - Adjustments - Desaturate (Removes colour information)

Add adjustment layer - channel mixer - check Monochrome box - 
percentages of sliders must add up to 100%

Add adjustment layer - hue/saturation - saturation to -100% - 
add second hue/saturation adjustment layer - change blend mode to colour
 - adjust sliders for effect