There are 75 colours available - 15 colours with 5 shades each.
Critters have 4 sets of colour and shade genes, and each part is inherited separately, the hue or darkness being somewhere between the parents'.

(image, two sets of parents and some offspring)

You can often see the resemblance between relatives.

Salmon	Coral	Carnelian	Blood	Mahogany
Almond	Coffee	Toast	Cinnamon	Chocolate
Shell	Peach	Tangerine	Bronze	Sienna
Vanilla	Lemon	Gold	Goldenrod	Mustard
Gooseberry	Olive	Lime	Fern	Moss
Mint	Jade	Leaf	Emerald	Forest
Pearl	Duck Egg	Persian	Myrtle	Peacock
Ice	Aquamarine	Sky	Ocean	Indicolite
Cloud	Azure	Cobalt	Indigo	Navy
Mist	Pigeon	Slate	Steel	Dusk
White	Silver	Stone	Smoke	Jet
Thistle	Lavender	Heather	Grape	Raisin
Lilac	Wysteria	Amethyst	Royal Purple	Midnight
Dawn	Orchid	Heliotrope	Violet	Byzantine
Rose	Tulip	Fuchsia	Cherry	Cranberry

Markings

The Base and Eye colours will always appear on a critter's body, while the two Extra layers in-between can vary. You can selectively breed critters to have markings - such as stripes, underbelly or lacing - on the 3 non-Base layers, or make the Extra layers disappear entirely.
(image here, of a few critters)
Here's one critter with the same colours but different marking genes. Lots of variety is possible!
Be aware that due to body shapes, some markings are shaped differently on certain breeds, or may not appear at all - how can a featherless critter show lacing? They still carry those genes and can pass them on, though.

(table of markings here. just on one breed of critter)

Abilities

Critters can learn up to 4 physical moves or spells. The abilities are the same as those that griffs can learn - see Move List for details.
Up to 2 abilities can be inborn, meaning they're inherited from parents and the critter knows them naturally. Some types of critter found in the wild will always know certain abilities. Don't worry about getting stuck with undesirable moves though - you can train them to forget things, or use magic scrolls to teach them the abilities you want. Just like big griffs!
Unlike griffs though, critters aren't affiliated with particular magic elements. Any critter can learn any move, and they don't have physical or magic stats that affect its power. They're effectively interchangeable, apart from their coat genes.
Critters cannot fight each other, but while assigned as a griff's companion, they can help out in battles by providing small boosts to the griff's stats, and sharing their abilities. The griff will become able to use a move or cast a spell as if they knew it themselves. This can be useful if your griff hasn't learned many of their own yet, or their spell list is limited by element. Attaching an 'elite' critter with some powerful moves to an inexperienced griff is a good way to help them train against opponents.

Tip: (must add this later...) whenever a griff channels their critter and uses one of their moves, there's a small chance they'll learn the move for themselves. The stronger or rarer the spell, the less likely - too many runes flashing by! - but occasionally something leaves an impression in the memory...

Management

While a critter is kept on your island, you can freely:

Change their name
Teach them moves
Breed them with another of your critters (there's no stud system like griffs have)
Assign them to a griffin (one critter for one griffin) or remove them
Release them back to the wild (permanent! They'll never be seen again)
Send them directly to another Master as a gift (we'll check if they have space)
Put them up for sale (any price higher than 0 will automatically add them)

You might not want to receive random critters as gifts; perhaps your enclosure is getting crowded or you're busy with your own breeding lines already. In your Office you can change your preferences to only allow critters from friends, or bar anyone from sending you any.

Guide: Menagerie

Colours

Markings

Abilities

Management

Whoops...