Legend items in a table

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]

Grid[{{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, {11, 12, 13, 14, 15, 16, 17, 18, 19, 20}, {21, 22, 23, 24, 25, 26, 27, 28, 29, 30}, {31, 32, 33, 34, 35, 36, 37, 38, 39, 40}, {41, 42, 43, 44, 45, 46, 47, 48, 49, 50}, {51, 52, 53, 54, 55, 56, 57, 58, 59, 60}, {61, 62, 63, 64, 65, 66, 67, 68, 69, 70}, {71, 72, 73, 74, 75, 76, 77, 78, 79, 80}, {81, 82, 83, 84, 85, 86, 87, 88, 89, 90}, {91, 92, 93, 94, 95, 96, 97, 98, 99, 100}}, BaseStyle -> {FontFamily -> Helvetica}, Frame -> All, FrameStyle -> GrayLevel[0.85]]

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

Column[{primes, composites}, BaseStyle -> {FontFamily -> Helvetica}]

Legended[primesgrid, legend]

Legended[Grid[{{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}, {11, 12, 13, 14, 15, 16, 17, 18, 19, 20}, {21, 22, 23, 24, 25, 26, 27, 28, 29, 30}, {31, 32, 33, 34, 35, 36, 37, 38, 39, 40}, {41, 42, 43, 44, 45, 46, 47, 48, 49, 50}, {51, 52, 53, 54, 55, 56, 57, 58, 59, 60}, {61, 62, 63, 64, 65, 66, 67, 68, 69, 70}, {71, 72, 73, 74, 75, 76, 77, 78, 79, 80}, {81, 82, 83, 84, 85, 86, 87, 88, 89, 90}, {91, 92, 93, 94, 95, 96, 97, 98, 99, 100}}, BaseStyle -> {FontFamily -> Helvetica}, Frame -> All, FrameStyle -> GrayLevel[0.85]], Column[{primes, composites}, BaseStyle -> {FontFamily -> Helvetica}]]