It looks like you are using a YCbCr format of your camera. When you are previewing or writing to disk, the toolbox converts the frame to RGB since display and disk logging require RGB data. When you use step or getsnapshot, the toolbox does not, by default, convert the image to RGB since there arr easons why you might actually want the YCbCr data.
You can set the ReturnedColorSpace property of your videoinput or imaq.VideoDevice object to 'RGB' which will cause the toolbox to convert the frame to RGB before it returns it. You could also use the ycbcr2rgb function to do the conversion.
