Kat's and Masato's links give you all the information how it was done and why information about the spectral distribution is interesting. But why averaging and publishing an average colour?

Here's why from the authors:

We admit the color of the Universe was something of a gimmick, to try and make our story on spectra more accessible... We would like to point out that our original intention was merely an amusing footnote in our paper, the original press story blew up beyond our wildest expectations!

in plain English: We wanted to read our names in the popular press as well and not just in the scientific journals

Wolfgang