What is an easy substitute for black mustard seed? I am making broccoli and chickpea dal.

From Cooking Ingredients, by Christine Ingram: she says "Black mustard seeds have an initial bitter taste and a sharp, biting pungency;" "Brown mustard seeds are milder than the pungent black seeds." Both brown and black seeds are grown in India. The yellow seeds are milder. My guess is, leave them out, as long as there are other spices and aromatics.

Has anyone tried yellow mustard seeds? Do they "pop" properly when dry heated, like black mustard seeds?

[Black mustard seeds are such a key (and delicious) flavour in Indian dishes that I would get them if at all possible.]