Pretty colors inside the slide of my 1911.

When it was heat treated there was some cutting oil left on the surface.

That is where the metal vapor layer was less dense in the Ionbond process.
Light Diffraction in the particle layers cause the rainbow effect, similar to case hardening and the structural colors of butterfly wings and bird feathers.

CZ pistols have a similar color from hardening, so I am told and see in my stainless cz75b.

I don’t know...looks very suspicious.

There might be a problem and testing is in order. You may need to send that out to a special test lab to be evaluated. Testing by someone trained in BS should look at this. It may take months or even years to determine if this is a problem. BS or Ballistic Sampling is an art form in and off itself. I just happen to be very good at BS and I could help you with this.

Ian McCollum did a video on the Korth 1911, which sported the same rainbow inside the slide.