A recent yukata’s patterns are so colorful, however, only indigo and white were used for yukata before.
Yukata is originally worn after taking bath. That means people wore yukata in the evening.
That’s why in daytime, people wore indigo color of yukata because the color is not transparent.
On the other hand, the reason for the white ground for night is that it is beautiful at night on eyes.
More traditional pattern of yukata is still popular now.