They will fade to white before the end of summer if exposed to
sunlight and that's a conservative time frame. Some fade completely in a few weeks. HD had some that faded in days but because they are used in a sandpit it doesn't matter as fresh color comes up as soon as the sand is disturbed.

The above is true WITHOUT the caustic nature of Portland.

I have used several different colors, red, blue, green, tan, orange. They do not give much color to the item even using white portland. A brushed look might bring it out some. I have had some red, white and blue stars out during the winter and the are still the original colors but a bit harder to tell. The backs are rougher so they show it a bit better but they have not been exposed to the sun. Wish it would have worked out better and brighter. Might try some coloring with it.
Paws

Hi Paws
Some brands are better than others for keeping there color. The Home Depot stuff goes white right in the bag. I heard the Walmart stuff was better and darker.

Have you tried brushing with vinegar to remove the cement haze/film that hardens on the surface of the aggregate?
I use either a soft steel brush or a brass brush, depending on the hardness of the concrete and pickling vinegar (it's stronger than the regular stuff) and brush the surface and rinse a couple times. It can make a huge difference to the color and brightness of your aggregate.