My take of the Alpha Legion is that they attempt to look like normal Marines rather than the spikes and tentacles of the other traitor legions. Their subversive activities mean they need to pass as normal marines.
We are marines from the Hydra Legion, now where is your central defence hub?Their equipment is going to be a mixture of old and new, picked up as they go along.
This is exemplified by this Terminator.
The body was sprayed with Citadel Incubi Darkness and I have started to paint the black base coats on the weapons, cables etc.