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]

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

Legended[primesgrid,legend]