To sign rainbow, take your dominant hand with the four fingers extended and your thumb tucked in. Start with the dominant hand's four fingers touching the four fingers of your non-dominant hand as the latter remains stationary on its side of your body, hand held upright with palm facing you. Move your dominant hand up in an arc, then down toward the other side of your body.
The sign is like painting a rainbow in the sky with your four fingers.
Rainbow is an ideal sign to teach through the flash cards, so your baby will recognize the sign when they see it in real life.
The rainbow flash card is great because it prepares your baby for seeing the real thing.