There are a squillion ways to change the colour of items, and no two photos require the same method.
In this case I'd add a Solid Color layer of maroon colour, set to "Screen" blend mode, and mask it on very carefully.
Hang on ... I just realised you posted on the wrong instructions. Have you been FOLLOWING the wrong instructions????
You're supposed to be following these instructions: https://www.damiensymonds.net/cal_S5X_pc.html
Well, it might suit you better to scan the whole page, then crop each photo out into four separate files afterwards? That's what I do sometimes.
Yes, you need to get your ass into the Levels Class!!!!! It's VITAL for these.
Gee, this is a worry.
Can you go to Photoshop's Color Settings, and make sure they're still on "North America General Purpose 2" the way they should be?