I, also, tested my Flintlocks for spark color (it was too cold to go outside so I had time). The results are as follows:
.50 cal TC Hawken White Sparks
.45 cal Pedersoli Kentucky White Spark
.40 cal Buggy Rifle Maker Unknown Orange sparks
.50 cal Traditions Trapper pistol Orange sparks
As a control, I swapped the flints on the TC and the Buggy rifle and the spark colors remained the same.
So I agree with No Powder it's all in the frizzen.