It makes the eyes bleed. But I have some tips for you, my friend.
1) Do not use default Paint colours. Invent your own blues.
2) Use pre-existing shading. What I mean by this is, notice where Torterra is a darker green than normal? When you recolour, make that darker green a darker blue than your normal blue. You'll be wanting to use either the paint bucket or you'll want to zoom in, grab the pencil tool, and recolour pixel-by-pixel.
3) ONLY do recolouring for now. Edits will come later. Hone your skills with recolouring pixel-by-pixel. When you get the hang of that, and only when you get the hang of it, start getting adventurous (though I'd recommend putting different pieces of Pokemon together before customising parts). At that point you should have a basic understanding of shading, shaping, and outlining.
4) Bla bla Fan Sprite forum bla bla.