The other day, to cut a long story short, due to my stupidity lost a few PSDs that I needed. I recovered them today with a free program called Recuva, it's all gone well until that point, I found the files and recovered them as told, but now the PSDs can't be opened by photoshop as they are an invalid file type despite still being a psd. The same thing applies with 2 png files, nothing wants to open them. Ideas why this is?

Probably some sectors of the files were overwritten before recovering them so now they are illegible.

Did Recuva mark these files in green as to say they were fully intact or yellow to say they are partially overwritten?

I think they were marked with yellow, so does this mean there is no way of getting them working?

That means portions of what used to be the file have been overwritten with other data. I don't know of a way to recover them further.