Nick has stated the correct way to mow bermuda. A little liquid fertilize really helps brown spots green up guick.NickN said:The problem is,you're cutting the bermuda too high.The taller leaves are shading the undergrowth.No,bermuda doesn't just green out on top.This is simply a result of cutting it too high.Cutting lower allows more sunlight in to the stolons,which in turn allows new roots,leaves, and stems to grow.Shading this area results in a thinning of that layer and less leaf growth,so when you cut,you see areas that have no new leaf growth.You want lateral growth with bermuda,not top growth.
Now,what to do?
Begin the season mowing bermuda 1.25-1.5" and keep it at that height as long as possible.(Usually July here) When you start getting less active growth in the bermuda(late in the season),raise your cutting height by 1/4".This will "hide" the slower growing undergrowth.Leave your mower at that level until you start to see the thin patches again.Then raise your blade another 1/4".By the end of the season,you'll be about 2-2.25".
Hybrid bermudas are generally worse at this than common.Reason being,hybrid bermuda grows thicker and more lateral(what you want it to do) than common.Common tends to stay green at lower stem levels simply because it doesn't grow as thick as hybrid,and in turn doesn't shade the undergrowth.