For this tutorial I am going to use an image from Wikimedia commons by Sebastianjude.
Open your image in Photoshop.
Duplicate the layer
Turn off the original picture layer by pressing the eye icon for that layer in the layers palette.
Select the duplicate layer and click the icon at the bottom of the layers palette to add a layer mask.
With the layer mask selected, choose the brush tool. If you have not used a layer mask before you will notice that the colour palette has changed to white and black. Black will hide the image and white will bring it back /reveal the image.
Use the paintbrush with black selected to hide all parts of the image except for the parts where you want to change the colour.
Once you are left with just the part you want to change colour. Turn the other layer back on.
Select the layer with the layer mask and choose Image > Adjustments > Hue & Saturation (You must select the layer on the left of the layers palette and not the layer mask this time). Play around with the colours until you are happy with the look of the image.
If you find a bit you’ve missed you can click on the layers mask in the layers palette and continue to use the airbrush and black to hide more of the top image. Likewise if you what to reveal more of the top image you can do this by using the brush and the white colour to bring the image back.
Here is my final image, alongside the original.