I understood your explication perfectly. What I found hard to understand about the chart at first was how, when you look at the top most or left most row/column (the ones that represent the parents), there would be, for example, a square with a 1 and another with a 2 that were right next to each other, but were different colours, even though they were different numbers. But I only found this confusing until I understood that the colours on this chart weren't being used to represent the different genes, but instead to represent the outward appearance, or the dominant gene of the budgie. Maybe that's what you found confusing too?This is a punnet square and as Rick says is used to show the outcomes of various pairings. The influencing parents genetics are shown in the left most and top rows. The remaining squares show which of the mutations the chicks may end up with.
So the first square shows that if both parents hold only the normal recessive white based gene then you can expect all the chicks to to be normals. Where as the second square introduces the Yellowface I gene. This square shows that the we could expect half the chicks to be normal and the other half to show the dominant Yellowface I gene and be split for the normal recessive white based gene.
I hope that makes sense.