No, I wouldn't trust box top art, because firstly, it is an artist's interpretation of the colour. Then you have the same image being reproduced by a printer, though the operator should be trying to match the artist's proof.
What you choose to represent Dunkelgelb is your choice, so in that regard if you prefer the colour on model box, then go with it. On the other hand, if you want to match historically what has been found through research, then you will likely end up with something else.
The current opinion is that there were several shades of this colour (five, with first one being a yellow used for farm equipment). This though was not by design, but as result of adapting to shortages, specially by the end of 1944 due to the constant bombing of German factories.
Please excuse, but the comp at work won't allow me to post a hot link, but here is a good read if interested:
http://www.network54.com/Forum/47207/thread/1364651560/A+%91big+question%92-+Dunkelgelb+%96+did+it+evolve-
regards,
Jack