1. open a Photo
2. Go to Select > Color Range
3. Click on the color on the picture you want to show in the actual picture. In this case i want to show the red dots on the mug. Drag the Fuzziness slider to the right to select more of to the left to select less. Then hit OK.
4. Go to Select > Inverse to inverse your selection
5. Add a Black and white adjustement layer
6. You can mask more out parts out if you want, using the white brush on the Layer mask.
Just try it out. But remember that selective color is something of the eighties -:)