[Excaliber]

So you got the yellow "not good" spark, thats a general rule. I've seen yellow spark that snapped loudly and jumped far and worked good. Blue, loud, jumps far is BEST!

Weak spark: Air gap that triggers your MSD or whatever is to close, thus a "weak trigger". Something wrong with the trigger mechanism in general, this one is very tough to diagnose.
Bad coil wire, to much internal resistance.
Bad dist cap bleeding off current by internally shorting, look for black little "arc" trails leading from one (or more) of the plug wire terminals. OR the center terminal. Weak coil, coils DO have a specific OHM reading you can measure for THAT coil. Out of range on OHMS, replace coil. LOW VOLTAGE to the positive terminal of your coil due to bad\dirty connection AT the coil or the wire that COMES TO your coil. Faulty bad ignition box, like MSD or Mallory. VERY tough to diagnose.

Key switch faulty not allowing enough voltage to pass to ignition system. The "rotor" in the center of the distributor is shorting straight DOWN to the metal distributor piece it sits on! I've seen GM HEI rotors literally burned through in the dead center of the rotor!

Old school point systems:
Burned points! Gap to small, gap to big. Condensor is bad, bad connections, low voltage, resistor that lowers voltage from 12 to 7 or 8 volts reducing voltage TO much, or not at all! Full 12 volts burns up the points in short order!

Good strong spark from the COIL, but WEAK at the end of the spark plug wire? Bad wire or bad "delivery system" TO the wire!

...I wish there was a simple straight forward answer!