Thank you eishiya! I hope this one will be accepted.

I want to learn how to work with limited palettes because I think it would help me being more aware of what I'm doing, rather than having too many colours to chose, and getting lost at some point.

Thank you also for the edit page trick, I deleted the old Slime.

I think I get it, no unwanted clusters or new coulours generated by tools unless you keep the same level of control on the whole piece by cleaning up.

Well I didn't expect such details, thanks for taking the time to enlighten me.
I agree I may have too many colors, the thing is that I learnt to make pixel art by myself on Photoshop, with layers and what not, so when I'm not happy with the colours, I can't help but give it a hue/saturation, or a colours balance. I also like to add a light noise effect on the final piece (I know this sounds like a bad idea), so that's why I have so many colours. But I didn't use any brush tool.
I didn't know the vision of the pixel art to PJ was so extreme, I just like to make nice pictures and I thought some pixel arts I made would fit in with all those amazing pieces I saw. 

I guess I'll try to restrain myself with some 16 coulours palettes and practice. I'll be back, thanks again for your answer, eishiya.

"Any art you post should be PIXEL ART! Pixel art implies that each pixel is placed by hand".
Don't worry, I can read the rules and I'm not making any kind of hybrid digital painting and pixel art. You're just making things up because you don't like the colours I chose. Your comment isn't constructive at all. Please, teach me how to make the "purest" pixel art, master :)