Ellan not super hard wearing I imagine - think nail polish (plus the end result will be matte, not shiny). I would do this to decorative parts that are not weight bearing / under constant friction. My satchel has a decorative buckle (real closure is via a hidden clasp) so the paint works perfectly there. If you get to it, protect the leather with cling wrap / sellotape / pieces of paper. The painting itself is really easy with the felt tip pen, just like coloring.
Matte silver looks awfully pretty against vintage leather - think tanned legs and silver sandals 
BTW a good quality leather belt is a great investment. I have two leather belts with steel buckles (tan and mahogany) that felt quite expensive upfront. But I've worn one or the other for most days during the past 4 years and they feel and look better with each wear.
Re: style day, go for it if you feel like the idea. It's definitely worth your time to find out your basic style preferences. I've never done the style day - there are lots of free online questionnaires and style guides - I did a few of them and put the looks I like on Pinterest boards, read a few great style guides (You Are What You Wear, Wear This Now, Nina Garcia's The 100). Then I sorted out my wardrobe (two piles: Love and wear often - Meh/ wear rarely) and analyzed what was common among the pieces in the "Love" and "Meh" piles. I think I really know what suits me and what I like to wear now. Things to take into account: your body shape, your work/ lifestyle, the message you want to convey with your looks, your peer group/social reference group (do you want to blend in or stand out), climate, and, of course, your personal esthetic preferences.