Looks like the sheet is using the built-on colour index. The mapping for these is in the source of openpyxl.styles.color
COLOR_INDEX = (
'00000000', '00FFFFFF', '00FF0000', '0000FF00', '000000FF', #0-4
'00FFFF00', '00FF00FF', '0000FFFF', '00000000', '00FFFFFF', #5-9
'00FF0000', '0000FF00', '000000FF', '00FFFF00', '00FF00FF', #10-14
'0000FFFF', '00800000', '00008000', '00000080', '00808000', #15-19
'00800080', '00008080', '00C0C0C0', '00808080', '009999FF', #20-24
'00993366', '00FFFFCC', '00CCFFFF', '00660066', '00FF8080', #25-29
'000066CC', '00CCCCFF', '00000080', '00FF00FF', '00FFFF00', #30-34
'0000FFFF', '00800080', '00800000', '00008080', '000000FF', #35-39
'0000CCFF', '00CCFFFF', '00CCFFCC', '00FFFF99', '0099CCFF', #40-44
'00FF99CC', '00CC99FF', '00FFCC99', '003366FF', '0033CCCC', #45-49
'0099CC00', '00FFCC00', '00FF9900', '00FF6600', '00666699', #50-54
'00969696', '00003366', '00339966', '00003300', '00333300', #55-59
'00993300', '00993366', '00333399', '00333333', 'System Foreground', 'System Background' #60-64
)
11L corresponds to 0000FF00 (hexadecimal) for which the rgb tuple would be green (0,255,0).