It would be nice to have an accurate measure of the temperatures on the black coated face and on the other one.
You want the top face white because you think it will reflect heat.
But other factors need to be considered.
Most of the heat could be transfered by convection or conduction.
If the face is black it could also absorb more (heating more the stone) and irradiate more, depending on its temperature, or more precisely it will irradiate at a different wavelength.
To summarize we need to know what's more important, reflectivity or emisivity.
Firstly, if you look through more of my posts you will notice I have talked quite a bit about carbonizing or seasoning the top. It was in this particular case I only wanted to carbonize the bottom. Secondly, there is no question that a darker surface will absorb heat more rapidly than a white or reflective surface, so I'm not sure why it matters what the temperatures are. Also, simply saying "temperature measurement" doesn't mean much anyway. Measured how, IR thermometer or thermistor? An IR thermometer will only measure radiation, and a thermistor will only measure conduction (assuming a silvery metallic probe).
Other factors need to be considered, indeed. Here are two of them. Metal conducts heat better than air or stone/ceramic, and a broiler element gets exponentially hotter than the stone. So if you aren't preheating your stone with a broiler element, and you aren't planning to preheat very long, or you use the broiler element during the bake, the top of your oven will be much hotter than the top of your stone. Depending on the preheat time and where the stone is placed, there is great potential for more thermal energy reaching the top of the stone via radiation from the top of the oven than from the stone itself. If you question this, just look at the top of your crust sometime. All that browning is a result involving convection and radiation; not conduction. Now look at the pockets that form on the bottom of the crust during baking, and take note of how pale they are. The surface of those pockets are only millimeters away from the stone surface. You have to realize that the pizza itself is a big heat sink. It's drawing so much energy from the stone via conduction, that the emission of radiation is diminished. So when you compare radiation from the stone versus radiation from a broiler element or preheated metal surface, the stone loses almost every time.or more precisely it will irradiate at a different wavelength.
Wavelength and temperature are not related in the manner you suggest. Bodies emit radiation over a broad range of wavelengths. To understand the full relationship, read about the Stefan-Boltzmann Law and spectral radiancy. However, carbon is the main actor in all oven radiation no matter what surface we're talking about, so spectral radiancy is unlikely to change significantly in any case.