Ditto on getting a color temp meter, or renting one. Sekonic C-500 is what I use (~$1,200, and $1,300 for a Pocket Wizard model.). The Lee filters are cheap to correct them at about $6 per 22" square sheet out of the local stage lighting house.
Power levels, age of bulbs (going warmer) come into play, as does the modifier. I know my small Nikon SB-900 speedlights run way cold, maybe 6,300K where the studios run around 4,900K on low power and 5,300K on high. I would have thought Nikon would have designed a better light and color, but they even shut down after a few full-power shots and Nikon screwed up there too (Common thermal shutdown issue with the SB-900 which led to the SB-910.).
I have an old Vivitar 285 speedlight which is still working and reads 5,200K and it has the yellowish-looking (filtered) flash window too. Guess they built them better back then for color balance with film, and now let digital fix whatever comes out of the clear speedlight windows now with camera's Auto-White Balance and fixing in PS.
SG