Ten colours, one per card, each filling the screen while its name is spoken. Colour naming is usually one of the first vocabulary sets a toddler masters, and a full-screen block of colour with no competing detail makes the word easy to attach to what they are looking at.
Most children match colours before they can name them, often around two, with naming following between three and four. Matching first, naming later is the usual order.
Blue and green, and pink and purple, are commonly confused well into the preschool years. Showing them one at a time rather than side by side reduces the guessing.