TO do it correctly you would need to replicate what is there currently. Baring that "red oxide" is accepted generally. You could use DP74LF tinted a bit darker or DP74LF tinted darker & a satin clear over that, not too sure how well the clear will stick.
What I am planning to do is use a PPG topcoat tinted the "correct" color & flattened. This should give better protection from the elements & the "correct" look.