I'm making dal for the first time and have all of the ingredients below, but I can't decide whether to use white, yellow (vidalia), or red onions.  I have all three in stock.  Here are the other ingredients:   1/4 cup ghee  1 1/2 cups diced onions 2 tablespoons minced garlic 2 jalapeno peppers, cored, seeded and minced 1 tablespoon cumin seeds 2 cups orange lentils 2 tablespoons ginger powder 2 tablespoons Garam Masala powder 1 tablespoon salt 1 teaspoon ground black pepper 1 tablespoon sugar 1 bay leaf 3/4 cup diced tomatoes 1/2 tablespoon rice vinegar 8 cups chicken stock  
My mum and I've always used red onions all our lives and everyone or a huge population in North India uses red onions. I have friends here in Australia from South India who I've seen using red onions as well when making dal. Having said that, I've also tried brown onions a few times and it doesn't make too much of a difference. And good luck with your first time making it :) 