This problem might lead to unwanted color differences. Putting two grass tiles of nearly identical colors together results in a checkboard pattern. It may be that in the beginning one uses colors with to much saturation. One tile in your drawning program looks very nice, but with many being put together in the isoeditor the tile looks to loud. It takes a lot of patience to create a well fitting groundtile, but to see it fit well gives you good satisfaction.
Sprites are all other elements as there are apples, flowers, walls, weapons, etc.
Like tiles they can be created with the help of a drawing program or by hand. The template ensures the correct perspective and size. Then the colored sketch gets scanned. Another way is to scan the original sketch and colorized it with the help of a drawing program. It is a question of one's taste.