You can have custom textures by renaming in an anvil a barrel with their proper name. For this you can find below a table with the value to put to rename it and the different possibilities for the type of wood, etc...
Barrel
Bookshelf
The different possibilities of customization.
Format to use:
<hoops type> <wood type> barrel <content>
​
Don't forget to remove the brackets before renaming your barrel.
Example: iron oak barrel apple
​
HOOPS TYPE
iron
gold
copper
​
WOOD TYPE
acacia
jungle
oak
spruce
birch
dark oak
crimson
warped
moonshine
​
CONTENT
apple
black
blue
brown
carrot
coal
cod
cyan
gray
green
light blue
light gray
lime
magenta
orange
pink
potato
purple
red
salmon
white
yellow
The different possibilities of customization
Format to use:
<empty> <wood type> bookshelf
​
Don't forget to remove the brackets before renaming your barrel.
Example: empty oak bookshelf
​
WOOD TYPE
acacia
jungle
oak
spruce
birch
dark oak
crimson
warped
moonshine
Don't forget that you also have to rename a barrel to get the empty bookshelf texture. This does not work by renaming a bookshelf.