Help me colorise this old photo of my mom using PhotoShop Elements 2020-how do I fix the green hair?

Yes, definitely watch 99Jon's video tutorial for a fuller explanation of the tool.  As I think you have already found, this is a great new feature of Elements 2020 which can not only be used to colorize black and white images but also to easily replace colors in a color photo.

See if my YouTube tutorial helps.

https://youtu.be/lLWs_-czppc

Once you have made a choice from the 4 offered in the Auto mode, you need to go into the Manual mode of the colorization tool:

Essentially, you need to use one of the selection tools to select the area you want to change - in this case the green hair.  Once you have a selection, click on the droplet tool and then click on your green hair selection.   You then use the color palette to select a new color for the hair.  If the palette does not provide a suitable color, use the eyedropper to select an existing color pixel in the image.  I recommend zooming in to perform this operation and make sure you are clicking a pixel of the color you are looking for.  This will change the color palette and give you several choices.  Click on the palette and try out the different colors until you find one you like.

I made the following quick edit to your photo.