Where do I find gradient_colourize.png?

To get the correct colours for the sky at different times for the day, I believe I need a colorizeGradientFile. Looking at East Coast USA, there is one listed, as art/sky_gradients/default/gradient_colorize.png, but I want to find where that is and edit it for my map. I looked around and cannot find anywhere that the file is stored.

This is my map, but it is producing colours that I don't want, so I want to create my own version with my colour choices.

 

The file in question is located here;

 

Ok, thank you!