Changing Colours in Photoshop

Replace Color tool. To see this tool at work, select Image -> Adjustments -> Replace Color. Apply it to a duplicate of the background layer.

Read more:

Another Method:
Hidden under the default Brush tool is the Colour Replacement Brush.

Fiddling with this brush will allow finer control to replace a colour under the crosshair but within the circle.

Read more about the Colour Replacement Tool here: