Honestly I don't really get the MachineMan parallel. I mean from what have seen MachineMan's helmet gives the impression of the front of a sports bike. If anything I see Birth's helmet more as a streamlined and "Metal Hero" version of Ryuki and Knight's helmet.
I see the Machine Man resemblance. Don't focus on the colors, look at the helmet's shape. Block out the eyes and you see the same overall silhouette, with the sweeping curve. Birth emphasizes the curve shape more but it's definitely present in Machine Man.