Translate any rgb value to 256 8-bit colors and back. Use the 'color' value as a key to the array in the json file.

colors.json

["0,0,0",
 "35,0,1",
 "71,1,2",
 "107,2,6",
 "142,4,10",
 "178,6,16",
 "214,9,21",
 "249,13,27",
 "1,35,2",
 "36,36,2",
 "71,36,4",
 "107,36,7",
 "142,37,12",
 "178,38,17",
 "214,39,22",
 "249,40,28",
 "5,71,6",
 "37,71,6",
 "72,71,8",
 "107,72,11",
 "143,72,15",
 "178,72,20",
 "214,73,24",
 "249,73,30",
 "11,107,13",
 "39,107,14",
 "73,107,15",
 "108,107,18",
 "143,107,21",
 "179,108,24",
 "214,108,28",
 "250,108,33",
 "18,143,22",
 "42,143,22",
 "75,143,23",
 "109,143,24",
 "144,143,27",
 "179,143,30",
 "214,143,33",
 "250,143,37",
 "26,178,30",
 "46,178,30",
 "77,178,31",
 "110,178,32",
 "145,178,34",
 "180,179,36",
 "215,179,39",
 "250,179,42",
 "undefined",
 "51,214,38",
 "79,214,39",
 "112,214,40",
 "146,214,41",
 "181,214,43",
 "216,214,46",
 "251,214,48",
 "undefined",
 "56,250,46",
 "82,250,46",
 "114,250,47",
 "147,250,49",
 "182,250,50",
 "217,250,52",
 "252,250,55",
 "1,5,83",
 "36,6,83",
 "71,6,84",
 "107,7,84",
 "142,9,85",
 "178,11,86",
 "214,14,87",
 "249,17,88",
 "2,37,84",
 "36,37,84",
 "71,38,84",
 "107,38,84",
 "142,39,85",
 "178,39,86",
 "214,40,87",
 "249,42,88",
 "6,72,84",
 "37,72,84",
 "72,72,85",
 "107,72,85",
 "143,73,86",
 "178,73,87",
 "214,74,88",
 "249,74,89",
 "12,107,85",
 "40,107,85",
 "73,108,86",
 "108,108,86",
 "143,108,87",
 "179,108,88",
 "214,108,89",
 "250,109,90",
 "19,143,87",
 "43,143,87",
 "75,143,87",
 "109,143,88",
 "144,143,88",
 "179,143,89",
 "214,144,90",
 "250,144,92",
 "26,179,89",
 "46,179,89",
 "77,179,89",
 "110,179,90",
 "145,179,90",
 "180,179,91",
 "215,179,92",
 "250,179,94",
 "undefined",
 "51,214,92",
 "79,214,92",
 "112,214,92",
 "146,214,93",
 "181,214,94",
 "216,215,95",
 "251,215,96",
 "undefined",
 "56,250,95",
 "82,250,95",
 "114,250,96",
 "148,250,96",
 "182,250,97",
 "217,250,98",
 "252,250,99",
 "4,20,167",
 "37,20,167",
 "72,21,167",
 "107,21,167",
 "143,22,168",
 "178,24,168",
 "214,25,168",
 "249,27,169",
 "6,43,167",
 "37,43,167",
 "72,43,167",
 "107,44,167",
 "143,44,168",
 "178,45,168",
 "214,46,169",
 "249,47,169",
 "9,75,167",
 "39,75,167",
 "73,75,167",
 "108,75,168",
 "143,76,168",
 "178,76,168",
 "214,77,169",
 "249,77,169",
 "15,109,168",
 "41,109,168",
 "74,109,168",
 "108,109,168",
 "143,110,168",
 "179,110,169",
 "214,110,169",
 "250,111,170",
 "21,144,169",
 "44,144,169",
 "75,144,169",
 "109,144,169",
 "144,145,169",
 "179,145,169",
 "215,145,170",
 "250,145,171",
 "28,180,169",
 "47,180,169",
 "77,180,170",
 "111,180,170",
 "145,180,170",
 "180,180,170",
 "215,180,171",
 "251,180,172",
 "undefined",
 "52,215,171",
 "80,215,171",
 "112,215,171",
 "146,215,171",
 "181,215,172",
 "216,215,172",
 "251,215,173",
 "undefined",
 "57,250,172",
 "83,250,172",
 "114,250,173",
 "148,251,173",
 "182,251,173",
 "217,251,174",
 "252,251,174",
 "undefined",
 "undefined",
 "undefined",
 "undefined",
 "undefined",
 "undefined",
 "undefined",
 "undefined",
 "12,52,251",
 "40,52,251",
 "73,53,251",
 "108,53,251",
 "143,53,251",
 "179,54,251",
 "214,54,252",
 "250,55,252",
 "15,80,251",
 "41,80,251",
 "74,80,251",
 "108,81,251",
 "143,81,251",
 "179,81,251",
 "214,82,252",
 "250,82,252",
 "19,113,251",
 "43,113,251",
 "75,113,251",
 "109,113,251",
 "144,113,251",
 "179,113,252",
 "214,114,252",
 "250,114,252",
 "25,147,252",
 "45,147,252",
 "76,147,252",
 "110,147,252",
 "145,147,252",
 "180,147,252",
 "215,147,252",
 "250,148,253",
 "31,181,252",
 "49,181,252",
 "78,181,252",
 "111,181,252",
 "146,182,252",
 "180,182,253",
 "215,182,253",
 "251,182,253",
 "undefined",
 "53,216,253",
 "81,216,253",
 "113,216,253",
 "147,217,253",
 "181,217,253",
 "216,217,254",
 "251,217,254",
 "undefined",
 "58,252,254",
 "84,252,254",
 "115,252,254",
 "148,252,254",
 "182,252,254",
 "217,252,255",
 "255,255,255"]

translate.c

int color = ((b&255 & 0xC0)+((g&255 & 0xE0) >> 2)+((r & 0xE0) >> 5))&0xFF;