Nice job. I'm not a fan of the black mouth on the fire sprites though. Since it's considerably darker than any other part of the sprite it looks a bit like an endless void that we can get sucked into if we stare at it for too long. It also just attracts too much attention because of how much darker it is than anything else.

Yes, it's a very important thing to not break the 3 color rule. Thankfully you didn't use primary red for the inside of the mouth but you used white and black which would literally be impossible to use for the original Super Mario Bros. I'm not denying that that's a major limitation but I don't know how they will be particularly useful for a fangame unless it's a joke game or the like.

Here's an example in Super Maro Bros. 3:

[url]http://www.flyingomelette.com/...avafortress.gif[/url]

Here's a good short video explaining it in-depth:

https://www.youtube.com/watch?v=dQw4w9WgXcQ

Why the Rick Roll

now waiting for smw - all parts for animations