[quote=@Meta]
<Snipped quote by Realmatic>

I don't get what makes it very difficult. Don't I just imagine it?
[/quote]

It's not just imagination. When you imagine the whole cube to be red, that imagination must come with the reason that the cube should be red. That's what you use to make the cube red. But if you're converting an entire door, there's a lot of different colours that you must convert to different shades of red. That is several Realmatic transformations that are near-identical--except in boundaries. You have to hold all those different transformations in your mind. This is entirely possible because of how capabilities work, but it takes practise.