It is true that the color of the ballast (black or white) affects its temperature?

If it is an enclosed ballast colour will barely affect the temperature of the ballast, provided the paint films are the same thickness. Outdoors the darker ballast will get hotter during the day, but at night it will be the same temperature, as most paints are pretty much equal emitters in the infrared wavelengths.

A matt surface is somewhat better at radiating heat, but this really is not going to have any effect on a ballast that is mostly cooled by convection to the surrounding air, either directly or though it's mounting surfaces.

I think the ballast should never reach the temperature, where radiation would make any significant heat transfer. Mostly (at least on EU ballasts) the color is pure visual look thing (e.g. Philips being gray). Sometimes, the color is chosen so, it visually mask out things like rust or thermal browning, so such ballast "look better then other" after long term use, making the impression of better quality.
Sometimes the color is only the natural property of the material used for ballast environment protection. E.g. some fine sand might be used to increase thermal conductivity of the coating layer, so it might be thicker, so resist longer to the water. So then the color of the sand used will influence the final ballast look.

Now that you say it I wonder, since white reflects heat and black absorbs heat...hmmmm

Now that you say it I wonder, since white reflects heat and black absorbs heat...hmmmm

Yes and no...
How it behaves in visible spectrum (so what color do we see) is not as important, as these things are way too cold, to have anything to do with radiation here.
And if it is "white" or "black" in IR area, where it does matter, does not much depend on what is the color in visible part of spectrum. So something, what appear black to our eyes might be easily "white" in IR and vice versa...