Who is Skoll/Hati?
Skoll and Hati are two wolf deities/jotuns from Norse lore. Skoll "Treachery" and Hati "Enemy" both hail from the Ironwood, a part of Jotunheim where the jotunfolk live. Their mother is believed to be the Hag of the Ironwood, or Angrboda, and their father is thought to be Fenrir- though it never specifically states.
In UPG provided by Raven K, he states: "When Fenrir first ran amuck he went back to the place where he was born. It is not known what happened to him there, save that when he left his maddened devouring rage began in earnest and a woman from [there] had borne two wolf pups in the image of their father. In some accounts, the mother of Hati and Skoll was Angrboda herself, by Fenrir her son, though we may never know the truth of this.
When Fenris was chained, Hati and Skoll attempted to interfere. They were captured by rthe Aesir and Odin put them to use, in the same way he used the Midgard Serpent. Sunna and Mani had often been known to change their course or dawdle. So Odin put the two wolves into the sky as a way to keep the chariots, night and day, on time. Skoll was bespelled to chase Sunna's chariot, and Hati was charged with chasing Mani's chariot."
In the end, when Ragnarok begins, Skoll and Hati will be released from their eternal chase of night and day. During this time they will kill and devour both Sunna and Mani, bringing about massive destruction and serving as the symbol that Ragnarok has truly begun. After they devour the sun and moon, they will join in the fight with their Father.
Outside of this, much isn't known about Skoll and Hati. They were never traditionall worked with or honored, so much of this has to then come from UPG/PCPG from reconstructionalist and those who work with the Rokkr. Like their father, Fenrir, they are believed to be very ravenous and hungry. They are also believed to be very emotional deities, in that they express wrath and anger much more frequently than other deities in that pantheon.
Offerings for Skoll/Hati:
It is not common to work with Skoll or Hati. While it is believed that they do like to roam when not chasing Sunna and Mani, they aren't particularly known for working with anyone or giving any benefit to those who give them offerings. Like their father, they are unpredictable. However, many people (Rokkatruars in particular) still like to honor them.
They don't have any set or particular colors. It's been suggested that their colors could be red, orange, black, dark blue and white to represent the shades of the sun and the moon/night and day. If you're going to make an altar to them, I would divide it in half with the colors representing both wolves, and appropriate colored candles on either side. Since they are jotun, and the kin of Fenrir, blood offerings would be the most appropriate- I should think. Offering them things symbolic of wolves, or of the sun and moon would be good as well. Carving wooden figurines for the altar could work.
Invocation:
"The Sun Cart's wheels are ripe red fruit,
Let them sink on my teeth, run their juices on my tongue;
Let the chase I am set be a game that I play,
With laughter, leaping, with wit may I run.
The Moon Cart's wheels are pearly fruit,
Let them sink on my teeth, run their juices on my tongue;
Let the chase I am set be the prey I will hunt,
With knowing, keening, like Death may I run."