It can be a challenge to brush toddlers teeth.
Please do start brushing your daughter's teeth twice daily using a baby brush. You could use a fluoride free toothpaste for her till she gets used to spitting out the foam. Please use only a smear of toothpaste. equivalent to about half a grain of rice to begin with and graduate to using a pea sized amount of toothpaste once she gets used to spitting out the foam.
Toothpaste is not meant to be swallowed but toddlers usually do so in minute quantities hence it is important to use lesser quantities of paste.
I would advise you to visit a dentist to rule out any decay or decalcification(loss of calcium ) if the yellowness doesn't improve after regular brushing.
Hope this helps. Do reach out if you have any further queries .