Simple Query Expression

Hello, i am trying to build an expression that will compare two fields and
will either create new value or copy current value from one of the fields...

I have two fields in my query: "Flag" (Red, Blue, Green, etc) "Notes" (text
field)

What i want is to check both fields and anytime there is a record with "Red"
flag and something in "Notes" field it will put "Red Flag with Notes" in
expression field, and for all other records it will simply copy the value
from "Flag" field.

I am sure it's a simple query, i just wish i'd knew how to write it.