1st thing to say i never did this but just by looking at this i can say sometimes you go lighter than the examples.
2nd image your ground is lighter than the reference(left screen robots foot and ground oil stain), same for the chick on the beach that one shows it the most, i guess if you couldn't touch the brightness/contrast then i suppose it is fine, though i would find it weird that teacher would manipulate the original in that way to purposely throw off their students? (re-reading you are allowed to touch the bight/contra)
idk if i am cheating with my monitor but i sometimes when doing general web usage youtube/img search references ect, i put my monitor to 50%/50% brightness/contrast - so maybe i can see the errors more apparent than someone at 100%/100% so i guess that is cheating? odd how that worked out for me to help you.