Those colored screens are the kickstart modules getting loaded in. Getting past those is a good sign. Some Flash modules (Algor, Deneb), or, more accurately, their contents, can conflict with them.
I have a hard drive on my PPC SCSI chain that causes OS4 to hang during bootup after the kickstart is loaded. I ended up having to edit one of the files in SYS:Kickstart to disable all units other than 0 (my main drive).
