Just to clarify, Sword of Mana is from the days before crazy back-lit portable screens, so they needed bright colors. What is more important about this example is its simplicity, only two colors and it works. You could easily get rid of some of your colors and adjust the contrast accordingly and it would look a lot more crisp. These minimal graduations in yours create a similar effects to compression artefacts, which makes a dirty impression.
Keep it simple at first, instead of having many shades of little perceivable difference stick to the two extremes of your contrast-range here. You don't want your main ground-tile, used to fill larger areas, to be too busy.