Hi Adam 🙂 Your first replier may have hit the nail on the head here, but there's a bit more to it I think.
Once Siril has finished processing files using a script it outputs the resultant processed image to a file called 'result.fits' in the currently selected folder (the ones where your lights, darks, biases and flats folders are). If you then use the File / Open option within Siril to open that file, you can then utilise the other facilities within Siril to further process the file, such as Photometric Colour Correction and Autostretching the image, then you just do a File / Save As and change the file type to TIF, then open that file in Gimp for all of your lovely colours to shine out.
There's a really good initial look at Siril and Gimp on YouTube here: https://www.youtube.com/watch?v=U5VndjG819s
Having struggled to get good results out of DeepSkyStacker and Gimp, my first few tries using Siril after watching this video resulted in much, much better final images!