I think it would depend how much boron is in the supplement. If it's balanced, that means for every 400 ppm of potassium the Coral Colors B adds, it would also add 4.4 ppm of boron. Operating on that assumption, if you increased your potassium by 40 ppm, ostensibly the boron would increase by only 0.44 ppm, which is likely not a problem. But I'm not sure if it's wise to assume that the additive contains the exact ratio of boron to potassium that seawater does. It should be the same or close, but Red Sea doesn't confirm that.
On the other hand, even if the supplement contains 10x as much boron as it should based on natural seawater's ratio of boron to potassium, increasing potassium by 40 ppm would only increase boron by 4 ppm. This is getting close to double the level in natural seawater, but it still might not be a problem.
Randy's article on boron has a specific section on
boron toxicity. It appears that about 6 times normal levels (24 ppm) appears to be harmful to at least some invertebrates, although there hasn't been rigorous scientific study on the topic, so we should be careful when interpreting these results.
I know you probably don't want to buy another additive, but have you considered a potassium-only additive since you know your boron is where it should be?
Brightwell has a potassium additive that's only a few bucks on BRS.