I'm building the Tamiya T-55. The instructions mention the color as olive. I have lots of the same olive green as used on WWII US armor but I'm not sure this is the exact shade. Will US olive green work, or do I need a different shade of olive?

Don't worry too much about Russian green. There are plenty of variations in the color the Russians used for their T-55s.
A usually good color for modern Russian armor in the Tamiya range is NATO Green, XF67 if memory serves.
But some olive can do the trick as well.

Olivier

well Chuck

That's an endless matter indeed!

Anyway looking at the reference pics you can see that actually Russian green could have different shade

You can have a color which is close to an Olive drab...



http://www.fotovalkirumodelism.com/page.php?page=382

another one which is closer to an olive green



http://dishmodels.ru/wshow.htm?p=1790

As Olivier has rightly told you, you shouldn't be too worry about a "perfect color match"

Model air Valleyo Russian green #71017 could work even if is a little bit darker

If you use Tamiya colors you can also use NATO Green, XF67 (...but also XF65 field grey could be right) or, if you want to depict an "olive drab" shade, XF51 kakhi drab

Anyway, considering that Weathering Techniques tend to darken your base color, if I were you I'd lighten a bit one of these paints

cheers

In addition to what Olivier and Mauro have said, you might also consider Vallejo Model Air Camouflage Green- I find it to be a decent match for the Guards green parade color. If you are planning to weather the vehicle then it might be a decent color to start with as it is more vibrant than Vallejo Russian Green.