After many attempts, I found that gradient colour textures look better than assigning triangular faces with different materials.

For most of the assets like dock, lotus, stones and mountains, I used lerp nodes to create gradient colour textures in UE material editors.

For the butterfly, I created the shape texture with red and green channels in Photoshop, then set the colour in UE.

Waterfall

For the waterfall, I used two kinds of material, and then I modified them using material instances.

Another one is from the UIWS plugin, however, it does not work in UE4.27, so I created related materials in my project. The water looks very clean and fluid. Initially, it only works for the spline assets which are defined by the plugin system, then I modified the coordinates with new nodes to make it work in my project.

UIWS:

https://www.unrealengine.com/marketplace/en-US/product/uiws-unified-interactive-water-system

Character

I used a cloudy material assigned to the character’s body, and set the emissive colour to make it shine in the dark.