I want to use Filmic color space for my renders, but I also want to override Filmic's constraints in the Compositor for the final image.
My current workflow right now is to open my Filmic renders in GIMP to adjust the brightness and contrast to my liking, but attempting to do the same with Blender's own Brightness/Contrast node in the Compositor doesn't allow the colors to leave their muted state.
In other words, if the color white was in my Filmic render and I wanted to raise the brightness to a full white, the Compositor doesn't let me get brighter than the dull gray white that's already in the render, so I have to either open the image in GIMP to have full control over the brightness and contrast, or switch back to the harsher and less accurate colors of the Default color space.
How can I get around this so I can keep my workflow entirely in Blender?
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TL;DR SOLUTION: While using the Brightness/Contrast Node is limited to Filmic's usual muted colors, the Color Balance node, when set to the drop-down option "Offset/Power/Slope (ASC-CDL)," will let you manipulate the colors fully.
I've found the two controls highlighted in red to act most like brightness and contrast settings, but it's better to read how to fully use this node here.