
Step 1: Open the image you would like to m=work on. Apple +J.
Step 2: Click on the brush tool. In the options bar, switch the Blend Mode to Color. Change your foreground color to black.
Step 3: From the brushes palette, choose a medium sized, soft-edged(0% hardness) brush.
Step 4: Start painting. As you paint, the color will disappear leaving your image in grayscale.