Sorry this reply took so long...
Technically, characters aren't in public domain...stories and images are. Those stories and images usually define the characteristics of what we think of as a character. So if the first stories or images defining a character are in the public domain, then you can create derivative works of those stories and images. In other words, you canuse a character if their FIRST appearance is public domain. If their third appearance is public domain, but derivative of their first appearance, which is under copyright, it becomes impossible to tell if another derivative work is really based on the third appearance or the first. So they'd have a case against you. Now, if there was an episode of gumby in the public domain, where Gumby turns into a pink frog, and it was public domain, you could use the pink frog, because it is not derivative of copyrighted episodes of Gumby.
As far as I know Gumby and Howdy Doody's first appearances are under copyright (actually, I'm not sure about Howdy)).
If you have questions about any other specific characters, post your questions on these boards and we'll give you our best guess. Each character's specific history is different...