So you want to learn how to recolor, huh? Recoloring is one of the fundamental parts of spriting, allowing you to do more complicated forms. So let's begin. For the purpose of this tutorial, we are just going to recolor a pokemon with a different pokemon's color. First, pick the pokemon you want to recolor and the pokemon with the colors you want to recolor it with. I picked Kakuna to recolor and Metapod to get colors from.
Now, zoom in so you can see what you're doing.
Now, before you even start recoloring, I recommend making a table. Later on when you get good at this you can skip on this step, but for now I highly recommend it. Basically, you pick all of the colors from the pokemon you're recoloring and then match up the color from the pokemon you're getting colors from. Since pokemon are shaded relatively the same, it shouldn't be too hard to match them up. However, if you find one pokemon has more shades than the other, you can usually use the shade above or below it as a replacement without too much harm, or you can create your own shade (though this is a bit advanced). Here is my completed table:
Now, you could manually replace the color with the fill bucket and/or the pencil tool, but who wants to do that? Instead, you can do this neat little trick. First, take the eyedropper tool and pick the color you want to replace. Next, repeat with the color you want to replace it with, but this time right click. Your current colors section should look something like this, with the first color on the top and the to replace with color on the bottom:
Now, click on the eraser tool. Just trust me. Now, instead of just erasing the sprite, RIGHT-CLICK as you do it. Now, only the color in the top section will be replaced by the bottom section. Neat, huh? This is how it looks like afterwards:
Nice! Now just repeat with the rest of the colors. Here's what it looks like when that's done:
You're done! Now just crop around the finished product, save it NOT AS A JPEG, trust me, it will ruin the quality (I recommend PNG), upload it, possibly transparentizing it in the process if you wish like at iaza.com, and enjoy!