There are several ways to simulate hand colouring of black and white images using Photoshop, so I have been using different methods to see which is best.
Conclusion - depends on the subject and the image.
Starting with a black and white image.
Add a new layer
Set the MODE to COLOR
Pick a oft paintbrush
Make sure the BRUSH is set to COLOR
Set the opacity to around 60%
Pick a color and brush on the layer
These lilies started life as a pinhole b&W film image and were hand coloured on Photoshop using this method.
Starting with a colour photo
Open the color version
MODE - Adjust_ Desaturate (or better - select a B&W adjustment layer if you have a later version of PS)
Use the History Brush on the adjustment layer to restore the colour
OR create a layer mask and pant with black on the layer mask.
These were done using that method