Is there a way to change the 5 colours of the trail renderer from script?
No, there is not, and that is extremely lame.
Here is the workaround I am trying - create a new material in Strumpy Shader editor that interpolates from one color to another color over the material’s UV axis and expose those colors as variables, leave the trail renderer’s colors all white with the alpha fading out the way I want it, then set the material colors by script.
In the Inspector under Trail Renderer you should see something labeled colors. Click this and five white boxes should drop down. Click the boxes and change them to what ever colors you want in you trail! Hope this helps!
-Rov