In all liklihood white. Royal brides used to wear bright colours for example, Queen Mary married in purple. But Queen Victoria, at a time when the custom was to marry in yellow or blue; started the trend for white dresses.
It is likely to have a train for effect on the tv cameras. If she confirms it will have sleeves. However, I can see her going sleeveless. A group of knitters from Middlesbrough who have knitted a balcony scene, have predicted sleeveless but, if they are wrong they are aiming to have a new dress knitted before Meghan is out of St George’s chapel.