The red text means that you are holding a weapon with a difficult above your current "effective power throw." The values Paul has provided in this thread and others are the minimum throwing proficiency required to have a given effective power throw in a given combination of armor. From the values Paul has listed, it appears that the calculation for the required proficiency can be approximated as:
Required Throwing Proficiency for an Effective Power Throw Level = 13 * Desired Effective Power Throw Level + Throwing Armor Weight
where
Throwing Armor Weight = head armor weight * 6 + torso armor weight * 2 + leg armor weight * 2 + hand armor weight * 8 - 10
with the caveat that the maximum effective power throw a character can have must be less than or equal to their power throw skill level. I think it is also safe to assume that throwing armor weights less than 0 count as 0.
Another way to present similar information would be:
Maximum Effective Power Throw Level = TRUNCATE[(Throwing Proficiency - Throwing Armor Weight) / 13]
It would save me a lot of time if some throwers could comment on the accuracy of the above equations.