The Vallejo colors aren't entirely inaccurate, there are just a few that are slightly off. Many of them are made to go in threes, with a base, shadow, and highlight, and don't look good by themselves. Later in the tutorial I'll teach you about filters and how to change the tone of a color after you've applied it.
Dupes - you'll still be best off with Vallejo, nobody else is any more accurate. You just need to ask around, because you wouldn't know that 'US Uniform' is actually the Vietnam-era color and not appropriate for WWII. Nobody should worry too much about getting spot-on color shades - temperature and tone of the real uniforms would vary with production batch, wear, and dirt.