See photos below

I painted a bulls-eye on the side of my mailbox for the snow plow driver.

What a pleasant sight - nice flowers around your mailbox! Just paint the post white/off white.
Unless you want to step up the design of the post and/or mailbox a little bit:

Lol Mark....do they hit it more or less now?

Well . . . I now have a post office box.

For the post I'd go with your taupe-y gray trim color, perhaps your shutter blue for the box.

...and the dog alerted us to the giggling teenagers loading our latest new mailbox in their trunk on a late Saturday night about a month ago. Emailed my PD and DPW that Monday morning to describe it, assuming they'd find several by the roadside somewhere. It was delivered to me ten minutes later, and (re-re-) re-installed by the end of the day. It has stories to tell.

I have a question about my mailbox and mailbox post. What color would look great? My house is gray colonial with darker gray shutters.

Barn red;)

Would be hesitant to just paint over mildew. Be careful to fully remove it. If you leave any chlorine behind it will use the residue as it starts to grow again. I like white but the green accent of the shutter is a nice accent color. So I might suggest that on the mailbox stand not or the box.