Bonjour Radhika !

That's a good question!
It depends which rule you follow regarding adjectives:

- if you consider the adjectives one by one (longs/blonds), therefore you respect their established position: long is always before the noun, and blond after, hence Elle a de longs cheveux blonds.

- if you link these two adjectives with et (and), therefore the rule is to place them after the noun, hence Elle a les cheveux longs et blonds.

In French you can do either, as long as you stay coherent with the rules.

I hope that's helpful!