Fortunately very irregular problems like power systems tend not to be huge.
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100's or 10,000's not billions of points
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Thus can use very difficult optimized "full matrix exploiting zero structure" algorithms
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In PDE case, one can show for some elliptic problems that red black can be better than original ordering
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For some hyperbolic equations used in computational fluid dynamics, this is not so clear and SOR methods are used
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If one has more complex differencing schemes e.g. fourth order differencing, then red-black does not work but a scheme with more colors (and more update phases) can be devised
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Difficulty is irregular graphs not complex stencils
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