Selective colouring refers to the technique of having a single colour appear in an otherwise black & white photo.
To achieve this, duplicate the background layer (Ctrl + J) & make it black & white. Add a layer mask to this new b&w layer. Select a suitable brush size & paint back in whatever colour you want (make sure the foreground colour is set to black). If you make a mistake just switch the foreground to white & paint the b&w back in (you can press “X” as a shortcut). Don’t forget to zoom in to make the colouring as accurate as possible.
(Hint: ” [ ” makes brush smaller; ” ] ” makes it bigger).