Customise admin write page with prefilled custom field

I’m trying to figure out how I can have a pre-filled custom field appear on the post edit screen everytime I go to write a new post.

To say it another way, when I click on Write Post, I want a custom field to already exist with a name like ‘MyField’ and a value of ‘blue’ without me having to manually type it in everytime.

Does anybody know of a plugin that might do something like that?

Or maybe point me to the code that would do this?

Thanks