Normally I agree with you Antoine, but this doesn't make an ounce of sense. Heat is not conducted to the dome, so thermal conductivity is not a factor. Even if it was most castable refractories have a thermal conductivity at or above firebrick. As for color, most fire brick in the US is buff while every castable I have ever seen is a dark grey, so your analysis of the function color plays is also off.
Ok, let me try again to see if I can win you over
The fire heats the dome. The heat then travels through the thickness of the materials. That happens more quickly with the bricks.
To me the standard refractory concrete once fully heated, is almost white. See picture bellow.
Using true refractory bricks, redish color, not the buff one that you describe which are more designed to be in fire places to insulate the construction. Those bricks once heated stay fairly red.
Hope you now agree with me
P.S.: the red fire bricks are very hard to find in the US as it is a food grade type of bricks and are most often imported from Europe.