It's very easy to do this.
For those who don't, open your intended image
Create a copy of the background layer (Ctrl J).
You now have two layers. Making sure the top layer is selected, press Ctrl+Shift+U to desaturate (or use another method to make the image B&W)
Now the bit most people fear.... layer masks.
With the top layer still selected, go to the layers pallet. At the bottom you'll see some buttons. Hit the one with the small white cricle (nect to the fx)
You'll see a white "mask" appear to the right of your top layer.
On this mask, anything white will not allow the image below to be viewed. However if you paint black, this will hide the top layer allowing you to reveal the colour image below. Zoom in close and use an appropriate sized brush. Do not worry about making mistakes as it's very easy to fix using a mask.
If you perhaps go over the edge and reveal some of the layer below you wanted hidden, just paint white over the area you want to hide. Easy!
You can use shades of grey to reduce/increase opacity of what you reveal too.
Makes these "hand painted" confetti images very easy.
