What are Hybrids?A Hybrid is a flower which is made from crossing two other flowers (parent flowers). You can buy a selection of flowers from Nook, but there are also more which can only be gained by growing them yourself, these can also be sold for a high price to Nook.
How do I get HybridsThere are two important things that you have to do when trying to make hybrids. You should lay your flowers across your town in the way that maximizes the chances of getting hybrids and want to put the right flowers next to each other in order to make hybrids.
Layout
I have found that the best way to layout your flowers is by putting a flower in every other space so that it leaves gaps for hybrids to grow, and it also makes it easy to see where new flowers have grown, because as you will see below, 2 Red Roses could make a Black Rose or they could make another new Red Rose.

This picture shows an example of a flower patch of red roses. This will hopefully grow some black roses!
Parent Flowers
Ones in bold are hybrids which can't be bought from Nook
Cosmos:
Red + Red = Red or
BlackRed + White = Red or White or
Pink
Red + Yellow = Red or Yellow or
OrangeWhite + White = White
White + Yellow = White or Yellow
Yellow + Yellow = Yellow
Pansies:Red + Red = Red or
Purple Red + White = Red or White
Red + Yellow = Red or Yellow or
Orange White + White = White or
Blue
White + Yellow = White or Yellow
Yellow + Yellow = Yellow
Roses:Red + Red = Red or
Black Red + White = Red or White or
Pink
Red + Yellow = Red or Yellow or
Orange White + White = White or
Purple
White + Yellow = White or Yellow
Yellow + Yellow = Yellow
Black +
Purple =
Blue
Gold Roses: To gain this you must have a black rose and a golden watering can. When the black roses wilts, water it with the golden watering can, and the next day it will have turned gold.
Tulips:
Red + Red = Red or
BlackRed + White = Red or White or
Pink
Red + Yellow = Red or Yellow or
PurpleWhite + White = White
White + Yellow = White or Yellow
Yellow + Yellow = Yellow or
Black
If you try to mix any other two colours then the outcome will be one of the parent colours. For example, if you put a pink and purple tulip next to each other, they would only make a new a pink or a purple tulip. This can be another good way of increasing the number of a certain type of flower, by laying them in a patch of similar ones. (this does not work for gold roses)
Watering Tips
There is no advantage in watering healthy flowers, it does not increase its chances of making a hybrid nor does it make it less likely to wilt. The best, easiest and quickest way to look after your flowers is to go round once a day and water all the wilted flowers only.
Also, all flowers wilt apart from Golden Roses, so if you do leave your hybrids outside make sure you water them if they are wilted. The only way to stop them wilting is to put them in your house.