Bill wrote:

I haven't painted the back door yet, and for the last 10 minutes
have been considering painting it a pale green (Color_F10 in
SketchUp).

I assume a "semi-gloss" sheen paint would look appropriate against
the Zinsser white. Is there an easy way to tint the Zinsser 123 I
have on hand or should I just go buy a quart of the color I want off
the shelf (provided I can find it)?
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Why do you want to tint a primer?

The finish coat(s) are going to cover the primer anyway.

Lew