Well you could use two petunias one white and one red. Assuming the red is Homozygous
Allow them reproduce a seed and allow it to grow. If the petunia that grows from the seed is pink it could be possible to say that this occured due to incomplete dominance.
R-Red r-white. Genotype of parents RR-Homzygous dominant rr- homozygous recessive
r r
R Rr Rr
R Rr Rr
Progeny- All pink Rr.
Hope this helps.