Spark plugs

I was checking out the ignition components with the multimeter today, being as I never rode this bike before I did the rebuild, and it's getting near to its road test once I get it off the sorn thingy. Everything checked out OK so I decided to take a look at the spark on my shiny new Iridium plugs. I was surprised to find the spark was of a very feeble apearance, yellow in colour. So I tried the test again with an ordinary GKN of the recommended type, but non Iridium, the spark with these was what I was looking for, nice big blue spark. I wonder has anyone had any issues with Iridium plugs, or any preference from experience with them. I have only run the motor on the bench as yet, and it starts ok with either type.