Red eyes in dogs mean the same thing as it does for us, except for being over-tired...this isn't something dogs express with two eyelids....
But it usually means first and foremost, irritation - just like in us and is often a sign of allergies..... there may not be other outward symptoms but I have seen a lot of dogs with red eyes rub them - this would support allergy irritation.
They get red too when I let my dogs stick their head out the car window.....the y don't bother them, and are gone within hours, but nonetheless this is still irritation.
If it cleared up, thats great news.