Whenever I'm not sure about something in my closet, my first response is to pair it with black. A casual tank or a structured moto jacket paired with oxford flats keeps things fresh.
Did you get pants in a shade you really, really love? Do you have similar shades in your closet? Try a monochrome look with your new pastel pants. Remember our rule of three. When mixing shades of the same color, incorporate it at least three times to make it look intentional.
You know what your grandma never wears? Military influenced clothing. Throw some army green or combat boots in the mix.
Since pastels are pale (Captain Obvious saves the day!), add a bright. Maybe just a bright top or bright jewelry or you could go bright with everything save the pants. The closer to neon, the better.
The devil, the devil being not your grandma, is in the details. Think sky-high heels, funky sunnies (Although, maybe your grandma's funky. I don't know her.), odd ball jewelry, wild manicures and modern cuts. Just avoid cat sweaters or anything else too kitschy.