When I was diagnosed a long long time ago, they taught me that the red
syringes and red vial caps were U-40 insulin. They also taught me how
to boil the glass syringes (although I never did that at home, only
when they taught me in the hospital) and how to do urine tests, but
those I'm just including for hysterical background and not because
they're relevant to my question :-).

Then I learned that the orange syringes and orange vial caps are for
U-100 insulin. And they taught me to never mix up U-100 stuff with
U-40 stuff, and to always pay attention to the color. ALWAYS MAKE SURE
THE SYRINGE COLOR MATCHES THE INSULIN TOP COLOR, they told me.

But now I take U-100 Lantus that has a gray stopper on the top of the
vial, and U-100 Humalog that has a white stopper in a purple metal
ring on the top of the vial.

My syringes are still orange-capped.

Does the color code not apply to insulin anymore, because the only
stuff commonly available is U-100?

I know the last time I went to Europe that U-40 was more common there.
Is it still common? How do I tell the difference between U-40 Lantus
and U-100 Lantus, if both exist?

I think that U-500 is available for some purposes. Would it have a
different color top?