I don't think you can get really good reference on Halo in general. For one, Halo spartans and ODSTs are like a bunch of ragtag super-soldiers. Their armors can be as different as each snowflake. Look at Halo Reach, each team members in Noble team has their own set of armor and each one is colored differently. In a real military organization, they wear uniform. Each and everyone of them has the same set of uniforms. Getting reference on uniforms would be an easy task.
I suggest that you go with what you think fits you. As long as you get the final look of being an ODST, or Spartans, you will be recognized as such.