Zandronum

Hello!
I used to play Skulltag alot a few years ago and recently I wanted to give it a new try, installed it again, then noticed that it seems to be outdated, so I downloaded Zandronum (still have to inform myself whats actually the difference, it just seems to be the same with a new name...?^^)
My favorite Doom Games are actually Heretic and Hexen, but when I try to start them with Zandronum, the colors go crazy and it seems like it uses the Doom palette for those games. Interestingly, this does not happen when I drag and drop the file on the Zandronum exe, but only when I start Zandronum and then select the IWAD or when I try to join a server.
Any ideas what might cause this problem? Is it a known bug or did I miss any options? Thank you very much!

I can only imagine this could happen if you got doom.wad or doom2.wad in your zandronum folder, please doubblecheck that.

Are you trying to run heretic as a pwad for doom? If using doomseeker you might've configured it wrong.

Well I had it in my folder along with the other IWAD files because I used to have it that way back when I played Skulltag. Now I removed all the IWAD files from there and linked Zandronum to my other ZDoom folder where all the IWADS are placed in but the problem still occurs :(

Put only 1 Iwad in zandronums folder and run it.. does it happen again? -Run zandronum not doomseeker-

Same problem. I removed the ZDoom-folder-link and placed HERETIC.WAD in the Zandorum folder again. I double clicked Zandorum and it started to run Heretic instantly but with really messed up colors. I then tried it again via drag and drop and it ran perfectly :(

Hmmm hold shift and run zandronum.. choose heretic and ensure GL compitability is NOT checked.

Well if I hold the shift key and double click Zandorum (btw I'm using v. 1.1.1) it just starts as normal. The menu where I can deselect or select GL compatibility appears only when there is another IWAD in the folder. Pressing shift key does not seem to make any difference here. I put Hexen.wad in there as well and checked what you said, but GL compatibility was not checked.
I just tried to start the exact same HERETIC.WAD with the last Skulltag release and I worked perfectly. :/

Thank you for the suggestions you offered so far!