I had the same problem a month ago. I updated Java and decided to remove older versions of Java. However, this also removed the 64 bit version of Java and only kept the 32 bit (i used Java's online tool for this: https://www.java.com/en/download/uninstallapplet.jsp).
So when I went to the settings in the launcher, the memory allocation setting reseted. It said I only had 512 MB allocated. The max it would let me allocate was 1 GB. With 512 MB allocated, my launcher had the same white-screen problem. With 1 GB, the game was kind of unplayable (because of lag).
Later, after re-installing the 64 bit version, the launcher would again let me increase my allocated memory up to 3 GB. I set it to 2 GB, which is more than enough.
So Basically, make sure you have the 64 bit version of Java (obviously you need a 64 bit computer) and that the memory allocation settings are correct in the launcher, because updating Java reseted the settings for me.
I don't have a mac, but if your version of the launcher has a console open it when you start the pack (it should be under the settings button in the top left corner, then press show console). Then once it white screens, tell me what the final line says.
I have the same prob. I use a 10.5.8 Mac OS X. and except for Tekkit Lite, It's for all modpacks! (Even Custom.) I don't have a crash report yet, but I'll respond once I get it. I really like Technic for one of the modpacks, Diamond Dimensions. (And Tekkit) But all of them crash! plz help!