They can be used for both gas and diesel.  I do have some red cap connectors that I can switch out the yellows with for a bit extra.   If you are the only one to fill up the cans it dosent matter what color the cap connectors are. They all have the gaskets in them but I can not guarantee that they are all good.  I have 10 of them and none of the ones I have used have leaked.  If they do you can buy new gaskets for cheep.
Cheers