Oh dear, this is becoming a rabbit hole. I should have said “as little as 1 pixel” needs to change colour to make it non-uniform. Or you could simply use a non-uniform image to begin with. That’s any image in which not every pixel has the same RGB value.
The larger point here is that if you are going to make any progress in Trainz content creation, especially with PBR textures, you will need a decent graphics program. One that allows you to;
- manipulate the RGB and Alpha channels of colour images right down to the pixel level.
- handle Layers
- read and save images with formats like 24 bit JPG, 32 bit BMP, TGA and especially PNG.
- operate in different “colour spaces”, namely sRGB and Linear RGB space.
And of course you will need to know how to use the program, a learning curve in its own right.
I happen to use a very old version of Photoshop, so I describe things in Photoshop terms, but there are freeware programs like Paintshop Pro, GIMP, etc. that I think can do the job but they probably use different names for the same things. I hope you can translate what I mean if you don’t have Photoshop.
So! Assuming you have such a program, use its Colour picker tool to sample the uniform colour image. It tells you the RGB values. Let’s say it's a blue with RGB = 100, 150, 200. Now with that colour active, choose the Brush or Pencil tool. Change the RGB a little, for example to 101, 150, 200. Set the brush size to 1 pixel, paint that new colour somewhere in the image and save it. Even such a minimal change will make the uniform image non-uniform.