Diffuse map aka colour map is the most common kind of texture map. It defines the color and pattern of the object. Mapping the diffuse color is like painting an image on the surface of the object.
The animation to the right shows the diffuse texture on it's own used on the robot-turret-sentinel example shown to the left
and the texture map itself:
In order to make a more realistic looking model or adding more detail onto a texture. We can combine several different maps which can be obtained from several 3D applications and combined in the diffuse maps, this process is known as baking
Textures that usually get baked are Occlusion (AO), light maps, shadow maps, cavity maps, etc.
Sometimes the diffuse map texture does not need to be a very complex texture, sometimes the texture might be a collection of colours or even gradients
This above sample is just a 64x64 texture map and only 112 bytes, depending on which kind of model you have created this might be more than enough, scaling the uvs to those colour swatches would be enough even if they are not on the right scale, for this type of models this would work perfectly well.
Another way of using reduced diffuse texture maps we can use, are gradients, the following scene only contains a single png (8bit 256 colours) 64x256px gradient texture shown next to it and the best part is that it is only 1.26kb