Legend Items in a Table 

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X primesgrid = Grid[Partition[ Table[If[PrimeQ[i], Style[i, Bold, 18, RGBColor[0.77, 0.42, 0.16]], Style[i, "Text" ]], {i, 1, 100}], 10], BaseStyle -> {FontFamily -> "Helvetica"}, Frame -> All, FrameStyle -> LightGray]

Emphasize the primes in a table of numbers and add a simple legend to the result.

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X legend = Column[{Style["primes", Bold, 18, RGBColor[0.77, 0.42, 0.16]], Style["composites", "Text"]}, BaseStyle -> {FontFamily -> "Helvetica"}]

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X Legended[primesgrid, legend]

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