I've heard a couple different ideas on the orange/red color. Everything from international orange to chevy engine red (both modelmasters), or even possibly a mix of the two. The problem is that even looking at photos online the color seems to change based on the lighting. The leading edges look like a gunship or euro gray (also modelmasters) to my eye.
Looking around I found Hawkeye in Australia does some USCG aircraft, but not the HU-25. You might be able to get some 1/72 and use them anyway though. Additionally, the instructions might have some color callouts you can use as the oranges and whites, I believe, are the same across the Coast Guard. Also, check the Helos forum for USCG builds and you might find something on the colors.
Groot