Blender uses Filmic color transform(shortly with version 4.0 will use new version of it - AgX), that compresses light values and makes highlights visible so they don't loose detail and look more natural as well. This also makes RGB(1,1,1) look greyish. You just need a higher value to show white after Filmic transform. Try something like RGB(20,20,20) with Filmic/AgX:
Or you could disable Filmic/AgX transform by choosing "Standard" transform in the Color Management settings:
In this case, your renders will not use Filmic/AgX, but the color values will just be clipped at some point instead of compressed in a sophisticated and nice looking way, so everything might start looking real ugly in case of your renders based on scene referred light values, but if you are compositing images that are not 32bit EXRs in linear color space and start with sRGB images in the first place, that's probably what you would want.