If you are light enough that you can sit on a colander without smashing it, then that is great, sure sit on it.
What ever works for you.
My colander is small. It won't sit on the toilet seat and even if it would I sure would smash it. *LOL*
The one I have just fits perfectly right in the toilet. It has a handle too. Makes for easy shaking and flushing. I used to keep the stones. I have such a collection now, that I flush them after I inspect them.