An NTSC Agnus works fine with a PAL oscillator (and vice versa). Swapping the latter is only necessary for 100% timing compatibility when using external video equipment.
All 4x 64K RAM chips should work AOK, there's no problem in mixing them (as long as they're working and fast enough).
Blue boot screen is reported to indicate chipset failure, arbitrary color may indicate catastrophic RAM failure and/or chipset failure. (I got a purple screen once when the oscillator had popped out of its socket.) If you've got the spares I'd try swapping the core chipset, the CIAs, MPU and then the RAMs.
I guess you've made sure all socket contacts and voltages are OK? Any bulged/leaked capacitors? Check if any of the PALs or the RAM chips feels very hot, this can sometimes be a quick guide.
