For example, I was editing a photo where the subject has blonde hair. I wanted to tone down some yellow leaves in the background but this was also changing the colour of the hair. Is this possible? I am very new to photography and lightroom so apologise if this is something simple. Many thanks
For Lightroom adjustment brush can help you in this direction, you can select background and decrease (for example) saturation, brightness. Of course you are limited on what you can change with the brush.
To make more precise selects with brush select Auto Mask and play with the size of the brush and feather. On Windows I use scroll of the mouse to change the size and shift+scroll to change the feather of the mask. Pressing Alt key (on Windows) and click you can reverse/remove the masking.
If this do not help I will propose to use Photoshop and mask.
P.S. In my opinion Lightroom is not the right instrument for selecting hair