Color Doppler is a test which is done to determine the blood flow in the placental arteries and veins which is flowing to the baby. This test is not compulsory but is done when you are closer to full term or if you go beyond due date. It helps the doctor understand if the placenta is able to support the baby adequately or if the baby needs to be delivered. 3d or 4d is an ultrasound and this need not be done. It's more of a fun scan where you get to see more details.of the baby and can even identify features.

Color Dopper is a kind of ultrasound scan which is advised to see certain specific aspects. But it is not a must- do during pregnancy. Go for it only if your gynaecologist recommends it.