I agree with growth. The skeleton calcifies, the corals grows new tissue and then zoox is last to join the party. This is why tips are white or different color as the body since it takes the zoox a while to find the Oregon Trail to settle the new land.
Burnt tips is when the skeleton calcifies and then the coral cannot grow tissue fast enough to cover it. This only really happens when your building block levels are VERY low (like much lower than seawater at .1N and .005P). Calcification is enhanced with low N and P levels, but if you are too low, then the coral cannot grow new tissue fast enough. This only really happens when using media or chemicals to artificially lower N and P - even 1 ppb of phosphate is enough, but you can get below this with some LC, GFO or organic carbon. I have .1N and .005-.01P and burnt tips are never really a problem. People who ride N and P lower than this (ZeoVit, for example), keep alk around 6.0 to limit calcification so that it will not outpace the tissue.
Low, but detectable, N and P levels will lead to this nice white or different colored leading edge growth pattern. If you grow slower, then the tips will be the same color as the corals since the zoox can keep up with slow growth.
All of this said, some acros just don't have different color tips no matter what you do (GARF Bonsai comes to mind), but you can sure burn the tips.