Can't really say I've seen it on Seiryu but red does seem to be the colour leaves adopt under sun.A lot of maples have varying degrees of redness depending on exposure but it also doesn't seem consistent one year to the next.
I must admit my Seiryu has had a red tinge to it for quite a while now but was green.It's not in a too sunny spot but there are reports of early autumn colours appearing here already...I think my Higasa Yama may be turning also so I don't really know.As you say it's healthy so I think I'd just enjoy it while it lasts :)