Here is the list of crossbreeding outcomes/how to get certain hybrids:

Random Plants:
Dandelions: Randomly grows in town
Dandelion puffs: Let dandelions wither and they'll become puffs
Jacob's Ladder: Randomly grows in town with perfect town status
Four-Leaf Clover: Rarely grows in town, usually gets destroyed like withered flower when picked up

Tulips: Red White Pink Red White Red Yellow Purple Red Yellow Red Red Black Red Yellow Yellow Black Yellow   Pansies: Red Red Purple Red Red Yellow Orange Red Yellow White White Blue White   Cosmos: Red White Pink Red White Red Yellow Orange Red Yellow Red Red Black Red   Roses: Red White Pink Red White Red Yellow Orange Red Yellow White White Purple White Red Red Black Red Purple Black Blue Purple Black Wilted Black Gold Can Gold

Flower Selling Prices Dandelions: 40 Dandelion Puffs: 50 Jacob's Ladder: 90 Four-Leaf Clover: 80 Red tulips: 40 White tulips: 40 Yellow tulips: 40 Pink tulips: 80 Purple tulips: 80 Black tulips: 240 Red pansies: 40 White pansies: 40 Yellow pansies: 40 Orange pansies: 80 Purple pansies: 80 Blue pansies: 240 Red cosmos: 40 White cosmos: 40 Yellow cosmos: 40 Orange cosmos: 80 Pink cosmos: 80 Black cosmos: 240 Red roses: 40 White roses: 40 Yellow roses: 40 Orange roses: 80 Pink roses: 80 Purple roses: 240 Black roses: 450 Blue roses: 2,500 Gold roses: 2,500

Interesting/useful facts:
Dandelion Puffs are used as equipment rather than headgear/accessories.
Black/Blue flowers are generally less common than other hybrids.
Roses are put in your mouth rather than your hair when used as an accessory.

Tips for crossbreeding:

Plant crossbreeding flowers diagonally - If you plant crossbreeding flowers diagonal to each other, there are more spots crossbreeds can sprout from than when planted next to each other.

Caring for crossbreeding flowers - Try your best to water the crossbreeding flowers whenever possible. If they wither, you'll especially want to water them then so that you can bring them back to life.

Where to keep crossbreeds - The best place to keep them would be your house, because in there, they can't wither. However, if you decide to keep them outside, you should at least have them in your house at night until morning the day after. This way, it will be impossible for them to wither.

Don't Time Travel - As tempting as it is, it can be dangerous when trying to create hybrids. You wouldn't want your crossbreeding flowers to wither, would you?

Garbage and items laying around - Try not to leave items or garbage on the ground. This may affect how likely a crossbreed hybrid is to sprout.

Crossbreeding crossbreeds - In example, say you want more than one of a pink tulip. You can not only crossbreed a red and white tulip, but you can crossbreed a pink tulip with another tulip and still have a chance at getting a pink tulip. If you crossbreed a pink tulip with a pink tulip, if a flower sprouts from that, it's sure to be another pink tulip.