A single peony and a densely double one can share almost the same colour but look very different in a planting. Look at the entire open flower rather than judging from a tightly cropped photograph.
Single forms show more of the centre. Fuller forms put more emphasis on layers of petals and the outline of the bloom. Neither description tells you the plant's mature size or whether it is herbaceous or woody.
Ask to see the whole plant
A close view of one flower does not show the stems beneath it. Check the expected height, spread and any support the nursery recommends. The right choice for a cutting border may not be the easiest fit beside a narrow path.
Compare flowers at a similar stage of opening. A partly open double bloom can look more upright and compact than the fully developed flower.
Keep the cultivar name with your photographs. If you are choosing several plants, write a simple planting list that includes flower form, colour and mature size. That gives you a design you can repeat, rather than a collection of attractive pictures with no reliable names attached.
Related reading: Peony Varieties Explained by Petal Layers and Flower Shape.