Hi Sonoma,
Yes, it is possible to write a GeoTiff file with color and alpha channel data using geotiffwrite function in MATLAB. However, you need to correctly define the "ExtraSamples" TiffTag in order to do so.
From the error and warning messages you received, it seems like the "ExtraSamples" TiffTag was not defined correctly. Instead of defining it as "Unspecified", you can define the non-color channels as "ExtraSamples". For example, you can modify the "TiffTags" struct as follows:
TiffTags.Photometric = Tiff.Photometric.RGB;
TiffTags.BitsPerSample = 8;
TiffTags.SamplesPerPixel = 4;
TiffTags.ExtraSamples = Tiff.ExtraSamples.UnassociatedAlpha;
This sets the "Photometric" tag to RGB, "BitsPerSample" to 8 (assuming your image has 8-bit color depth), "SamplesPerPixel" to 4 (indicating that there are 4 color channels), and "ExtraSamples" to "UnassociatedAlpha" (indicating that the extra channel is an alpha channel that is not associated with any specific color).
Then, you can pass this "TiffTags" struct to the geotiffwrite function like this:
geotiffwrite('img.tif', img, R, 'TiffTags', TiffTags);
This should allow you to write a GeoTiff file with color and alpha channel data using geotiffwrite function in MATLAB.
