How to darken a light compact powder?

You can't really darken the powder itself, but you can apply the powder, and then apply a bronzer to warm up your skin tone.

Try using a bronzer over it very lightly all over your face. Make sure not to apply too much otherwise you could end up looking cakey and orange! Just enough to match your neck and the rest of your body.

you could try rubbing the brush on a darker palette. For example, put the dark one next to the light one, and take your brush and brush back and forth like it is one big palette. I have an ELF palette...it actually came with a whole bunch of different shades like a big square of different shades. putting your light one next to a dark one and rubbing back and forth will even it out, but you may need to experiment to get the right shade.

Another option is to find the receipt and go back and trade it. Next time you go, test the palette on the backside of your hand.

No, the best thing you can do is return it and get one that matches you.