while planting flowers in Animal Crossing New Leaf, putting too many of the same flower in one area doesn't look great, so I tend to combine different flower types and colors to create an interesting effect.
My personal favorites are:
Pink tulips and blue pansies (Great for Re-Tail)
Yellow tulips and blue pansies
White and purples violets with white cosmos
Orange lilies and yellow violets
White lilies and pink lilies
Pink roses and purple roses
Red roses and Orange pansies
Orange roses and white carnations
Any black flower and any red or purple flower
I have yet to find and combinations with cosmos
What are some of your favorites?
My personal favorites are:
Pink tulips and blue pansies (Great for Re-Tail)
Yellow tulips and blue pansies
White and purples violets with white cosmos
Orange lilies and yellow violets
White lilies and pink lilies
Pink roses and purple roses
Red roses and Orange pansies
Orange roses and white carnations
Any black flower and any red or purple flower
I have yet to find and combinations with cosmos

What are some of your favorites?