Can you eat yellow flowers from mustard greens?

You can! They are edible. I cook the mustard greens sometimes with garlic and onion, or like you would steam spinach.

You can also add it to soups and stews, if you have other questions about which flowers are edible, the books "the edible flower garden" and "wild plants of *your area*" would be the place to find all sorts of information. The second book is what told me it was OK to eat mustard flowers.