The relationship between the forty colour index number you get from the
default colour palette when you press Fill Color and the background colour
values is given below if you swap the last two digist with the two first.

My query is: do apis exist that calculates the one given the other?

Sincerely,

Jan Nordgreen
Santa Cruz, Bolivia

aPalColour(1) = "000000"
aPalColour(2) = "FFFFFF"
aPalColour(3) = "FF0000"
aPalColour(4) = "00FF00"
aPalColour(5) = "0000FF"
aPalColour(6) = "FFFF00"
aPalColour(7) = "FF00FF"
aPalColour(8) = "00FFFF"
aPalColour(9) = "800000"
aPalColour(10) = "008000"
aPalColour(11) = "000080"
aPalColour(12) = "808000"
aPalColour(13) = "800080"
aPalColour(14) = "008080"
aPalColour(15) = "C0C0C0"
aPalColour(16) = "808080"
aPalColour(33) = "00CCFF"
aPalColour(34) = "CCFFFF"
aPalColour(35) = "CCFFCC"
aPalColour(36) = "FFFF99"
aPalColour(37) = "99CCFF"
aPalColour(38) = "FF99CC"
aPalColour(39) = "CC99FF"
aPalColour(40) = "FFCC99"
aPalColour(41) = "3366FF"
aPalColour(42) = "33CCCC"
aPalColour(43) = "99CC00"
aPalColour(44) = "FFCC00"
aPalColour(45) = "FF9900"
aPalColour(46) = "FF6600"
aPalColour(47) = "666699"
aPalColour(48) = "969696"
aPalColour(49) = "003366"
aPalColour(50) = "339966"
aPalColour(51) = "003300"
aPalColour(52) = "333300"
aPalColour(53) = "993300"
aPalColour(54) = "993366"
aPalColour(55) = "333399"
aPalColour(56) = "333333"