When making realistic looking sprites or large images for use with OpenRCT2, dithering is the best solution.

Wikipedia: Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as color banding in images.

• No palette:

  

• RCT2 palette, no dithering:

  

• RCT2 palette, dithering:

  

To dither images for OpenRCT2, two general approaches can be taken.

The sprite compiler that converts images to be used in game has options to automatically dither the images. For more information see Sprite compiler.

Images processed with an external tool will not be affected by any of the sprite compiler modes. If you want more freedom over the way an image looks, you can use an external tool.

• Dithering Images using Photoshop