After playing around with it a little, I was able to get a color image (although in a bitmap not JPEG) doing the following, where visaObj is an open visa connection rather than a GPIB. The SCPI commands and rest of the workflow should be the same though
% send command and get BMP.
fprintf(visaObj,':DISPLAY:DATA? BMP, COLOR');
screenBMP = binblockread(visaObj,'uint8');
fread(visaObj,1);
% save as a BMP file
fid = fopen('test1.bmp','w');
fwrite(fid,screenBMP,'uint8');
fclose(fid);
% Read the BMP and display image
figure;
imageMatrix = imread('test1.bmp','bmp');
image(imageMatrix);
% Adjust the figure so it shows accurately
sizeImg = size(imageMatrix);
set(gca,'Position',[0 0 1 1],'XTick' ,[],'YTick',[]); set(gcf,'Position',[50 50 sizeImg(2) sizeImg(1)]);
axis off; axis image;
