Hi Saloni,
We have never used a sucrose gradient to separate plant cell organelles. To separate mitochondria from other organelles we used Percoll or PVP-40 gradients (in described protocol we used 0 -4.4 % gradient of PVP-40). After ultracentrifugation in PVP-40 gradient mitochondria form a light yellow/grey band low in the tube (cloudy), chloroplasts form a green band high in the tube, whereas peroxisomes settle at the bottom of the tube.