This entry is from the lizzy FAQ added back in version 1.4 of the old book, coldblooded is as far as I know worded exactly the same so the following should still be right:
Q: When a unit with the Cold Blooded special rule is required to add or subtract additional dice for a Leadership test, how does it work? (p41)
A: Take the test on however many dice it would normally be taken plus the extra dice from the Cold Blooded special rule and remove the highest. For example if a normal unit would take the test on 3D6 and add them together a Cold Blooded unit would roll 4D6, take away the highest and then add the remaining 3 dice together.