Likewise I have tried a number of weapons...none of which have created a value in #27. Again, if you have a specific part, it would be appreciated.
I looked at some costume files of mine for a pair of characters that have two weapons (double pistols and claws). On both of them, #27 was blank. However their right hand weapon was never listed anywhere in the files and, in addition, when I loaded their costume file their right hand weapons was listed as "cannot find part". This may then be some sort of error in the costume creator where it is not correctly generating the right hand weapons. I'm going to manually edit in a #27 value as if it was the right hand version of their weapons in a little and give an update on what I find.
Edit for update: No luck
but I'm pretty sure that's what's going on though: the costume creator isn't generating the right hand weapon correctly. Luckily, as the colors are the same, all you should have to do select the correct right hand weapon.
A random side note on translating colors between hex and B/G/R- rather than looking for a translator you can also see if you have a graphics program on you computer already that could do the swap in a palate/color selector tool.