Is there a colloquialism for appropriate?

Yes, there are many colloquialisms for "appropriate" depending on the context. Here are a few examples:

General use:

* Fit for: "Is this outfit fit for a job interview?"

* Suitable: "That movie is suitable for all ages."

* Decent: "That's a decent excuse, I guess." (Sometimes implies a minimum standard, not necessarily ideal)

* Proper: "It's not proper to interrupt someone while they're talking."

* Right: "That's the right thing to do."

* Okay: "It's okay to wear jeans to the party, but dress up if you want."

* Acceptable: "It's acceptable to disagree, but be respectful."

Specific to behaviour or situations:

* Behaving oneself: "The children are behaving themselves today."

* Mind your P's and Q's: "You need to mind your P's and Q's when you're at the dinner table."

* Not going overboard: "Don't go overboard with the decorations."

* Keeping it real: "Let's keep it real, we can't afford that."

* Playing by the rules: "If you want to win, you have to play by the rules."

The most suitable colloquialism will depend on the specific context and the tone you want to convey.