I'm using MATLAB R2017a and rgb2lab accepts uint8 inputs. Is the Image Processing Toolbox on your version of MATLAB installed?

help rgb2lab

rgb2lab Convert RGB to CIE 1976 L*a*b*

lab = rgb2lab(rgb) converts RGB values to CIE 1976 L*a*b* values. rgb

can be a P-by-3 matrix of color values (one color per row), or it can

be an M-by-N-by-3 image array, or it can be an M-by-N-by-3-by-F image

stack.

lab = rgb2lab(rgb,Name,Value,...) specifies additional options with one

or more name-value pair arguments:

'ColorSpace' - Color space of the input RGB values.

'srgb' (default) | 'adobe-rgb-1998' | 'linear-rgb'

'WhitePoint' - Reference white point.

1-by-3 vector | 'a' | 'c' | 'd50' | 'd55' |

'd65' (default) | 'icc' | 'e'

Class Support

-------------

The type of rgb can be uint8, uint16, single, or double. The output has

the same shape as the input. The output type is double unless the input

type is single, in which case the output type is also single.

Examples

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[1] Convert RGB white to L*a*b*.

rgb2lab([1 1 1])

[2] Convert an RGB color to L*a*b* using D50 as the reference white.

rgb2lab([.2 .3 .4],'WhitePoint','d50')

[3] Convert an Adobe RGB (1998) color value to L*a*b*.

rgb2lab([.2 .3 .4],'ColorSpace','adobe-rgb-1998')

[4] Convert RGB image to L*a*b* and display the L* component as an image.

rgb = imread('peppers.png');

lab = rgb2lab(rgb);

imshow(lab(:,:,1),[0 100])

See also lab2rgb, xyz2lab, lab2xyz, rgb2xyz, xyz2rgb.

Reference page for rgb2lab

Make sure that you have used Matlab 2014b or later.

If not, you can download this function from the Matlab central "https://ww2.mathworks.cn/matlabcentral/fileexchange/24009-rgb2lab".

It works fine with version 2016a as well. What is the version you are using?