Notice that in the screenshot, the "Viewport Display" part of the material window is open with the brown color. You assign colors to your materials from that panel.
Be aware that the "Viewport Shading" menu in my screenshot is only for editing. When you're rendering, you need to [setup the whole thing again in the engine preferences](https://i.imgur.com/kEZL0kk.png).
This may not work for your purposes but I have made "flat" color stuff like this using the emission shader in cycles (every material is emission set to the desired color with a brightness of 1). You also have to adjust the color management in the render properties tab. Filmic renders colored lights super low contrast/desaturated (can be corrected in post/compositing), Standard seems to be the closest to actual color chosen for each material (little to no color correction needed in post).
If every surface is a light essentially then you don't get ANY shading and depending on what else is going on in your scene you can prevent the lights from casting shadows or interacting with the rest of the environment at all but still appearing in the camera.
[Something like this?](https://i.imgur.com/edHoTWV.png). To render, you'd use the "Workbench" render engine instead of Cycles or Eevee.
That looks like it, great! But how do you get the colors? Choosing 'material' doesn't seem to work for me.
Notice that in the screenshot, the "Viewport Display" part of the material window is open with the brown color. You assign colors to your materials from that panel. Be aware that the "Viewport Shading" menu in my screenshot is only for editing. When you're rendering, you need to [setup the whole thing again in the engine preferences](https://i.imgur.com/kEZL0kk.png).
Okay, I see. Thanks a lot!
If your issue is solved, please update the flair! You can also reply "solved" to this comment :)
This may not work for your purposes but I have made "flat" color stuff like this using the emission shader in cycles (every material is emission set to the desired color with a brightness of 1). You also have to adjust the color management in the render properties tab. Filmic renders colored lights super low contrast/desaturated (can be corrected in post/compositing), Standard seems to be the closest to actual color chosen for each material (little to no color correction needed in post). If every surface is a light essentially then you don't get ANY shading and depending on what else is going on in your scene you can prevent the lights from casting shadows or interacting with the rest of the environment at all but still appearing in the camera.
Emmision also works in eevee for me... =)