‘Unpremultiple’ is the process of dividing the alpha out of the image. Then the colour corrections can be done. After this stage, the alpha can then again be multiplied back in. This can often be done within the colour correction node itself, or manually. from Digital Compositing
Many 3D programs have a specific compositor node for this.
I tried to find an unpremult node in Blender compositor but I didn't find such a node.
How would I perform unpremult