My way of thinking is that the Chromosomes of the plant alter, to me its like atoms, say more atoms change from one to another in the plant and start to mix, and the colours start to change.
It is a bit scientific to explain,
If it is a good sport that you think is worth keeping, you have to look down the stem of the one that has sported and try and take some cuttings from the leaf joint, and root them.
It will take all winter to get them to root and make a small tuber, if you can do that you then have to plant the new tuber out in to your garden and hope that you have been able to keep the colour change.
I have had about 5 sports in about 40 years that we have saved, if you look at Pat & Perc below it is orange but next to it, it has sported pink, I could see that it was possible for that dahlia to sport as the inner petal tips have got pink tinges, I took about 6 cuttings from this last year 2011 and rooted all this last winter, this year 2012 we planted all 6 cuttings out and 4 cuttings made it and are now pink the other 2 have reverted back to orange, so now we have a new dahlia that we will be selling in 2014 as we have to build some more stock up of it.
Biology Chromosomes
A thread-like structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.
Each chromosome consists of a DNA double helix bearing a linear sequence of genes, coiled and recoiled around aggregated proteins (histones).
Their number varies from species to species: humans have 22 pairs plus the two sex chromosomes (two X chromosomes in females, one X and one Y in males).
During cell division each DNA strand is duplicated, and the chromosomes condense to become visible as distinct pairs of chromatids joined at the centromere.
Gateshead festival sported with me, Amber Festival. 
Pat and Perc sported with me named Pink Pat and Perc.
JRG Dahlias 