Leave Color
Use Leave Color to convert your image to grayscale except for a specified color or range of colors. This filter was popularized a few years ago by music videos and a few cola commercials where the entire scene is in shades of gray except for the product (or star of the video), which remains in color.
ColorSpecifies a color to remain untouched; the other areas of your image are converted into grayscale.
ToleranceDetermines the amount of variation from your chosen color to tolerate before it converts what remains into grays.
Feather AmountDetermines how smooth the transition is between areas that remain in your chosen color range and the gray areas.
Method