ColorSpace argument must be ''cmyk'', ''gray'', or ''cmykgray''

The toolbox enables converting color data from one color space to another through mathematical transformations.

RGB
The RGB color space represents images as an m-by-n-by-3 numeric array whose elements specify the intensity values of the red, green, and blue color channels. The range of numeric values depends on the data type of the image.

For single or double arrays, RGB values range from [0, 1].

For uint8 arrays, RGB values range from [0, 255].

For uint16 arrays, RGB values range from [0, 65535].




for more information

https://www.mathworks.com/help/images/understanding-color-spaces-and-color-space-conversion.html