changing a black and white photo to a color photo

Hi Ko's answer above will describes how to go from colour to black and white but to go the other way depends.

If your black and white image is a conversion from colour in camera raw then by going back to camera raw you can change the adjustments back to colour (i.e. the adjustments there are non destructive)

However if all you have is a black and white image then you will need to hand colour - in the same way that early photographs were hand coloured. Put a new layer above the image and change it's blending mode to colour. Then paint on with the colours you want. The Hue and Saturation will be taken from the new painted layer but the lightness will be taken from the black and white image below.
There may be some areas where you will need to use a mask and darken/lighten sections of the black and white image to get the final colour - but the principle of the colour layer and the underling luminosity (black and white) layer applies.

Dave

Hi

There's plenty of tutorials on Youtube

How to Colorize a Black and White Photo in Photoshop - YouTube

You got caught in the Photoshop Sketch (mobile) forum. (I'll move it to the right forum)

But here's one (SUPER FAST) way you can change your photo to black and white (there are many ways):

I prefer to use an Adjustment LAYER to control the Black and White change.  The Adjustment Layer doesn't change the original photo, so you can always go back and make edits later on.

Click the little BLACK AND WHITE COOKIE at the bottom of your layers, then choose Black and White.  You'll get options that let you make changes to the effect without ruining your original image.