Incomplete dominance is the expression of phenotype of two paired alleles (i.e dominant and recessive allele) all together.
Usually when two alleles get paired, the characteristics of the dominant allele is expressed while the characteristics associated with recessive alleles are expressed only when the two recessive allele get paired.
Here in this case the characteristics of both type of allele are expressed i.e. both orange and purple strips appear in the offspring. Hence, this case shows the incomplete dominance.