How much effect does painting a domestic radiator have? Yes, I know it should be called a convector :-)
The obvious effect is that a coat of paint, which is generally a thermal insulator, can limit the rate of heat transfer from the metal to the air. But how important is that effect to the overall efficiency of heating my house or room?
 A: If the convection efficiency is lowered by an insulating layer, the water returns hotter. I chose some ballpark numbers, under the (not really true) assumption that the heating is only convective:


*

*Power = 1000W  

*Area  = 1 m^2

*Thermal Conductivity = 0.2 W/mK (median of some plastics on the
internet)

*Thickness = 2E-04 m  (200 micron ballpark for paint)
Using dimensional analysis (because I don't remember the equation) I get
delta T (temperature drop across the paint thickness) in K:
[Thickness (m) * Power (W)] / [Area (m^2) Conductivity (W/mK)] = 1 (K) !
So a ballpark number would be a 1 degree K (or C) difference in surface temperature. 
If the room is cold (10C) and the radiator is almost too hot to touch (140F = 60C) then it's a few percent effect on the convective efficiency, roughly speaking, and probably no more than 10%.
But a radiator temperature of 60C and room temperature of 10C, emissivity of 0.85, and area of 1 m^2 gives about 600 W of outgoing radiation, and ~360 W of incoming radiation from the room, or a net radiation of 240 W, so really can't ignore either one.
A: Assuming your house is well insulated, the First Law of Thermodynamics applies. This means that the total amount of energy in your home should be constant. If you were opening and closing doors and/or had leaky windows those effects would outweigh paint coverings on an insulator.
As you mention, Radiation and Convection are different. Convection is heat generated from contact with the air. Radiation is generated from the radiator to you or other objects, without mediating through the air. This site discusses the various advantages of differing paint styles. It estimates about a 9% improvement if you use black paint. However, as mentioned on the site, metallic paints of any kind are a big no no. So don't use them. In addition, the paints used on the site are specifically designed for radiators, so you'll want to look for black radiator paint rather than just typical black paint.
For a more theoretical treatment looking at the ratio of the Stephan-Boltzman Law equations, look at the radiative power section, shows the same.
A: From the surface to room you have radiation and convection.
A dark color will have better radiation so if you chose a good radiant color then should have no loss (don't pick white).
A painted versus non painted should have the same convection.  
So the only difference is the conductive thermal insulation of the the paint.
Paint is pretty thin - not much insulation there.   
And you you need to consider the boiler does not do more work. The produced heat gets into the house. It just takes the radiator a little longer to release the heat and it is released at a very slightly lower temperature.  
Paint em.  The difference is very very small.
