I'd use Tab.Color instead of Tab.ColorIndex otherwise you'd also have to query the actual palette used by excel.
Completely untested code written on the fly, there may be typos / mistakes. Use MSDN to check the syntax of excel methods.
filepath = 'C:\somewhere\somefile.xlsx';
excel = actxserver('Excel.Application');
workbook = excel.Workbooks.Open(filepath);
numsheet = worbook.Worksheets.Count;
namergb = cell(numsheet, 2);
for sheetidx = 1:numsheet
sheet = workbook.Worksheets.Item(sheetidx)
namergb{sheetidx, 1} = sheet.Name;
bgr = sheet.Tab.Color;
namergb{sheetidx, 2} = uint8([mod(bgr, 256), floor(mod(bgr/256, 256)), floor(bgr/65536)]);
end
workbook.Close;
excel.Quit;
cell2table(namergb, 'VariableNames', {'SheetName', 'RGB'})
