I can't seem to control my cat

I know that's a broad title, but to give a bit of background, I adopted my cat from the humane society a bit over a year ago. She was in the house with my older cat, and she came from a house that had multiple cats. At the end of February of this year, I had to put the older cat to sleep because her kidneys had just completely shut down.

Since then, Corrigan (my other cat) has been scratching the walls constantly and crying at all hours. I can't get any sleep and no matter what I do to stop her from scratching the walls, it's not working. I won't get her declawed because I don't believe in it, and the watergun hasnt' worked. She has two scratching posts, but she still scratches at the carpet and walls. (The walls are the big issue). I keep her claws clipped down and give her tons of attention, but it doesn't seem to stop her from clawing. and it's not just one random wall it's 3 or 4 of them.

She starts crying late at night and just keeps going, running through the apartment usually about 4 - 5 am. I get up at 6, so this really kills my sleep cycle and I find myself not awake as much as I could be.

I'm not sure what breed she is, her coloring is not something I've seen on cats before (it's a light gray pastel shading/cream/blue/white/pink overcoat with silverish undercoat), so that's not helping much. I don't know if it's just that she misses the other cat, so much so that this is her way of grieving or what is going on. If any experts here can help, I'd appreciate it.

I'm no expert, but it sounds to me like she's lonely alright. Every cat grieves differently, and if she was used to playing with your older kitty this could be her way of acting out.
Several people have suggested Feliway plug-ins, to calm your kitty. This may work for the moment, but if it were me I'd be out looking for a new feline family member, ASAP !

(and yet another amateur opinion: I believe her colouring is what's called a dilute tortie or dilute calico)

I think she's lonely. Cats like humans grieve for lost friends and companions. I think maybe this is her way of telling you she needs you to help her through this. sounds crazy I know but I'm telling you, that's what it sounds like to me. Twig did something similiar to that when we Lost Zen a few years ago, he was very clingy and cried alot, woke us up all the time. Unfortunately we couldn't get another cat right away because Zen passed from FIP so Twig was our main focus for the year we waited until the vet said she thought it'd be ok for us to have another cat. I think we pretty much put our grief on the back burner for Twig( sounds crazy I know but my cats are my kids, the only kids I'll ever have so I treat them that way spoiled rotten!) and tried to be strong for him. When we finally got another cat(rocket) Twig hated him at first now they are inseparable! this is true with the Girls also, Isis and Luna, although they aren't as close as Twig and Rocket Twig still is caught giving them kisses once in awhile. Maybe you should look into getting another cat when you're ready. it might help in a tremendous way, I know it helped Twig. He's now so clingy and doesn't really wake us up much anymore. (and he sleeps with Rocket it's so cute!)

Yes I think she needs another kittie too.

It does sound like she would love to have another playmate. I would definately recommend one that is around her age. Also, S-L-O-W introductions help to make the introduction better. Here is some information if you decide to get another cat:

http://www.pawschicago.org/PetCare/catintroductions.htm

Good Luck

Katie