Nice job on the stamping. Coloring concrete is often a problem. As much as it might seem to be homogeneous, concrete is not. It varies in its density, water-cement ratio and other properties throughout the mix. We are usually only concerned with the average conditions, but in coloring, we don't have that luxury.
You are probably seeing some lightening with the bleeding of calcium compounds to the surface of the concrete. Try a small area spot test using dilute muriatic acid to see if the white haze disappears. If not, it is color dilution in the mix. That's more difficult to fix.
You can use a concrete stain to cover the entire area, but you might still see some splotchiness. You can also make a dilute mix of your colored powder and water, with just a small amount of cement and pour that on top and let is soak in.
Outside of those, you can try some of the dyes that are available and mix dye or tint into your sealer.