How to relax an hornet without it loosing its color?

[arfem]

Hi! I just received four Vespa orientalis specimens from a friend in Greece, it took about a week for them to come in the mail (I live in Italy), so they are all curled up and stuck that way. I need to relax them in order to give them a proper position for pinning. I tried to use a relaxing chamber with a Polistes dominula male specimen some time ago, but it turned out so dark that I almost couldn't see the pattern on its abdomen. 
I wouldn't want to risk wasting a specimen, as these wasps can't be found where I live. What can I do to relax the specimen without altering its color? 

Thank you in advance!