Any software or tool to create or convert to RGB

[reo99]

I am looking to create high quality heightmaps for a game that doesn't just use the common grayscale terrain.

Instead, this game uses the RGB channels to allow for 3x more detail.Generally, heightmaps go in steps like 0 0 0, 1 1 1, 2 2 2, etc.This game however does treat 1 0 0, 0 1 0 and 0 0 1 as the same, but allows you to go e.g. 0 0 0, 1 0 0, 1 1 0, 1 1 1, 2 1 1, 2 2 1, 2 2 2, etc. for much greater detail. I find that it is essential for smooth terrains.

How can I create realistic heightmaps using this method?

In case it is relevant or if you're were curious, it's a racing game and it uses data points every 5 meters. Maximum terrain height delta is 99m.