I'm very thin. Will eating brown/red/black rice and millet varieties help in weight gain?

We cannot say that it will help in weight gain, but it would certainly help your body get the micronutrients that it otherwise cannot get from eating white rice. 

When you eat food which is wholesome, living and plant-based (covered on day 6), your body starts getting re-aligned to its true nature and you'll reach your ideal weight (as per your body type) if you’re following the habits taught in the Ultimate Health Challenge.