http://www.ipmsstockholm.org/colorchart ... rts_us.htm
The easiest way to use this site is to get to the colour/colours you need, then Print Screen and paste into a new Paint window. Then you can grab a "swatch" paste it into the Paint window in which your drawing is being made, and use the colour sampler.
I used to use this technique with photos of aircraft, however with photos you are subject to the vagaries of weather, lighting, and perspective.