I bought new tires and they have like a white powder on them

[Riff Raff]

Anthrax powder for sure.

[IndianaVX]

X2 on the purple power, be sure not to get it on cladding or the paint. If you do, keep a wet towel close to wipe it off.
Spray the tire down, scrub it down, and then rinse it with water.
Just don't let it dry before rinsing with the water. and be sure to rinse any off the wheels that will have gotten on them.