1. Does it mean that if the flash unit can take GE No. 5 , then it surely can take Sylvania Press 25 or other bulbs that has boyanet base?
Any flash that takes a No. 5 will also fit a No. 6 or Press 25 or 26B

2. What is the different between M2, M3 or M25?
The M2 is the same size but has less power, and a slightly lower color temp. My first question that started this thread was how do the M3 and M5 or for that matter the M25 differ. So I canít say exactly.

3. Why don't we just use Blue bulb since it can be used when taking both B&W and color photo. Why is clear bulb needed since it is only used on taking B&W?
The blue reduces the light output. You get more power if you use B&W and clear flash. Also when flash bulbs came out it was all for B&W as that was the film at the time. You can use clear flash with a blue filter and color film or skip the filter and use blue flash.