If I set a key frame in source text to change the font and the color, the Essential Graphics window would warn that it's not accepted in Premiere Pro because it's "keyframed". How to make it possible? I'd like to set the text of the template in PP, but to animate the font and the color in After Effects.
To be clear, are you using the new text styling expressions? You can make the font choice user-selectable in the EGP - when you drag in a Source Text property, there's an "Edit Properties" clickable next to the text entry box in the EGP. Here you can choose what you want accessible to the user.
If you're building out a few "approved" typefaces for them to choose from, you'll want to do this using the Dropdown Menu expression control and some appropriate expressions coding.
There's some language in your question that's making it a little unclear, but there are other (easier) ways to set that color aside from those text stylishing expressions. - Create a text animator > Fill Color > RGB. - Use a Fill effect on the layer. - Use the Color Overlay layer style.
Each of these also give you the ability to animate the opacity of that color (if you wanted it to fade from one color to another, for example), but make the color accessible to the user, if that's what you're after.
Another option here would be to use those text styling expressions to set the specifics, and then have the user-editable text actually coming from another text layer. While it could be disabled/Opacity 0 in the actual comp, THAT would be the one whose Source Text is in the EGP, and then it's feeding into the visible text layer via expressions.
Let me know if you need more guidance on any of these ideas!