We could use a component that, when present in a scene, renders a fullscreen quad.

It should be configurable at what point it is rendered (pre / post tonemapping etc). Need a way to define the sorting order.
You can select which material to use.
It should expose a color or some other way to input custom parameters.

For maximum flexibility it may also be useful to be able to rotate and offset the quad, so that one can place blotches at different places in screenspace and rotate them around the screen axis. Then one would also need a scale/size value.

This can be used for:

• Full screen pictures (loading screen background)
• Fade in / out masks
• Some HUD elements