I was wondering if there was any difference between the shades of red used below the waterline on the ships of WWII? USN, DKM, IJN, and RN. I have viewed the different builds online and I have seen everything from bright reds to dark red-browns. Very confusing! Any help is appreciated Thanks

The red used on US, British, & German ships was a lead base red oxide primer. You can duplicate this using any red oxide primer from a hardware, or automotive parts store. I prefer the Duplicolor brand. Some brands are redish and some are more brownish in color. The correct color is the brownish red. Just spray it into a cup and pour it into your airbrush and you are good to go. I use this primer on my ships and when it is photographed it appears red.

Each Navy had a different formula with corresponding differences in their colors.Even the US Navy had different formulas for different types of ships; Check out the "Ship Bottom Paint" listing on the right column here:
http://www.shipcamouflage.com/paint_listing.htm

Ive heard that Pre-drednaught ships had brass sheeting laid onto the hulls, which would oxidize into a strange greenish colour..

Polly Scale Railroad Acrylic Colors-

#414293 - Zinc Chromate Primer (not the sickly green/yellow aircraft color)
#414350 - Mineral Red
#414350 - Special Oxide Red

If you like to make ship models, you will find these useful for doing the submerged part of the hull.

We have the antifouling reds (and other antifouling colors) of the various navies accurately matched and available in our Colourcoats enamels:

US 14, Norfolk 65-A Antifouling Red (USN WW2 & prior);
RN 19, Royal Navy Antifouling Red (WW2);
M 06, Modern USN Antifouling Red;
KM 04, Schiffsbodenfarbe III Rot 5 (Kriegsmarine WW2);
KM 05, Schiffsbodenfarge III Grau 1 (Kriegsmarine WW2 on U-boats, S-boats);
IJN 11, IJN Antifouling Red (WW2).

They're all different....

here is another site I find useful:

US Navy Camouflage and Colours

That source is both outdated and inaccurate, having been matched to house paints.