What you got so far looks pretty good...so just keep it up. A small note for future reference: at the bottom-right the dithering is rather redundant, as there's a fitting intermediate color in the palette.

An intermediate color, is one that is located between two keycolors in colorspace. Most importantly a color with a Brightness roughly the average of the keycolors, and preferrably with a Hue and Saturation close to the average as well. Or simply a color between two other colors in a decent ramp (like your orange colors).