You said you had talked about a lot with her before you disassociated. It sounds like whatever you talked about brought something up and it's still too hard to talk about.
I am wondering if your response to her asking you to pick out the colors that wanted to resist in anger was simply because whatever you zoned out about has anger attached to it that you buried and never sorted through. I have had that challenge myself, the challenge of anger that I could not articulate.

Yes, I think one can have an anxiety/panic attack if they are startled out of a dissociated state of mind.

Also, in trauma therapy work it's not unusual to feel flat or tired afterwards too.