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Doodals, can I see you in the office?
Medium-brown will maintain the color of your shoes as they are.
Dark brown will slightly darken them over time...but they will also develop a very nice patina with some color variation...(it won't ruin the appearance of your shoes in any way...and I can post pics of C&J's that have been polished only with dark brown to show you the patina...)
Black will darken them a lot...though they will never become truly black...they can become very close to it...you could use some a very light coat of black on top of the dark brown on the toes to give it a bit or a mirror shine and for that burnished appearance...if you desire this...but too much black will alter the color of the shoes significantly.