This is the result of a mutation in the cells that produced the flower. Every time a cell divides, there is a small chance of a "copy error" that will produce a mutation. You probably have at least two mutations in your DNA. That mutation can be passed on when that cell divides again. If the mutation winds up in a reproductive cell, the offspring will also have that mutation.

OK, so why won't this mutation be passed on? The mutation happened early in the development of the flower, and half of the cells in the flower have the genes to be yellow, while the other half have the genes to be orange. When the flower produces seeds, the seeds from the yellow half of this flower will carry the "yellow" genes, while the other half will carry the "orange" genes.