That is not white - it's pink. To get the CIE LAB values, using book formulas, then it depends if your numbers are floating point or integers. If they're doubles, you must divide by 255 (that's why the answer from @_ didn't work).
% If values are doubles, you must get them into a range of 0-1 by dividing by 255:
lab_pink = rgb2lab([255,128,128] / 255)
% If values are uint8 then no scaling is needed:
lab_pink = rgb2lab(uint8([255,128,128]))