If you're somewhat new to watercolors, colorful leaves are a great thing to paint. The way the colors naturally blend together as the leaf changes from one color to another is absolutely beautiful.
Make sure you have an actual leaf or a great picture of a leaf to use as reference. Use a watercolor pencil to sketch out an outline of your leaf on your watercolor paper. You can also sketch out a little bit of detail in the middle.
Use a larger paint brush to fill in the whole leaf area with the primary color of your leaf. Make sure this first layer is very watered down with just a little bit of pigment. While this layer is still wet, you want to go back in with more pigment and dab in some color. What’s great is that watercolors naturally flow and blend together in a way that’s very similar to how leaves actually change color. I added in some different shades of green, a little bit of brown, and a few orange spots.
Once you’re happy with the overall coloring of your leaf, let this base layer fully dry. Then go back in with a smaller detail brush (the one that came with the Prang watercolor set was perfect for this) and add in details like the veins in the leaves. I’ve found that instead of trying to paint every single line and vein, it’s better to just do some of them. You can also use watercolor pencils to draw in tiny details or create texture.