Honestly, very fun game! It was nice playing it and liked the fact you included two difficulties given the fact I like the game going rough on me in these kind of games. Really nostalgic for me-
The only main issue I have with it so far is that Colin's lack of invincibility frames makes him sometimes get caught up in the enemies' attacks with no escape. Perhaps make him have a recovery of sorts when this ends up happening or make him take more knock back every time he's hit while stunned? I don't know if you should make him unable to be hit by the same enemy after their combo while stunned.
An addition you can make is some kind of bar for heavy attacks. Basically: You beat up enemies and your bar gets filled up. Every time you do a heavy attack, you end up wasting a bit of the bar. Would be interesting if it ignores regular attack end lag, allowing for some brutal knock back after a standard combo.
Overall, really liked this game and haven't played a Mario Combat based game in years! I'm assuming you looked up games similar to the classic flash one, but in case you haven't, try googling up Mario Combat Deluxe and Power Star Game. I'm pretty sure they're all created by the same creator of the original. Might inspire you. (or hey, if you're just a random dude reading this, check 'em out anyway lmao)
Good luck on the project, and don't rush it! It seems very promising so far.
EDIT: Literally this came to my mind after like, 10 minutes of posting this. Oh well. Basically funni red screen goes br when you are getting hit. dat's it, poof-woof