Pentel does make 0.5 red and blue leads. I tried using them when I was working
as a book editor, but there are a few problems. As stated below, they are
indeed very soft and were constantly breaking--even the 0.7 sizes aren't that
strong compared to black lead. But also, whether 0.5 or 0.7, they are very
difficult to erase, which kind of goes against the rationale for using pencil.
I've always wondered why the properties of colored lead can't be made
comparable to black, but I suppose there's a good reason.

I can't remember the manufacturer (it might have been Pentel), but I did once
buy a multi-color mechanical pencil that had about a dozen different colors of
lead in it--naturally it was a big, fat, round pencil! But the leads were so
thick they were almost impossible to write with. I think they were meant more
for drawing.

Paul

Note: All the coloured leads I've seen are 0.7 or bigger - I think the
0.5 and smaller sizes break too easily and so are not made.

David