Well, in several searches, it looks like a gray-brown Agouti is the natural color of wild rats. Supposedly domesticated Agouti rats are a bit redder than wild rats.
"RSA standard for agouti-golden chestnut brown with dark gray to black base fur and black guard hair, black eyes"

The Spoiled Ratten Rattery has some pictures of domestic Agouti, as well as a pretty decent description of the color.

A fawn hooded rat (what I believe you are describing) is not something that would occur in the wild. Since they not camouflaged at all like an augoti is.

They sound like abandoned pets to me.

Thanks so much for your responses, I am relieved that these are pets, not wild. (of course, now more concerned for them and their well-being). The fact that two showed up within an hour, and that one is the tan/white, seemed likely they were pets. I am located in Ottawa, Ont area. I will try to see if I can get hold of a trap, although my concern is accidently trapping any one of the other animals and birds that come and go in my yard.
I can watch, for the most part, but I have some pretty bold squirrels and chipmunks here!

You can contact the Ottawa Humane Society. They should be able to lend you some humane traps. They will also take the rats ... although the conditions they keep small animals in are not the best, it is probably preferable to being outside. They would also stand a chance at getting adopted.

If I wasn't dealing with that nasty myco strain I would take them in a pinch.

Thanks again for all your replies. I cant believe I've spent the better part of the afternoon in a battle of wits with a rat, but I digress. ...

I set up a trap with all I had handy until I can get something better. Basically I propped a small budgie cage up at one end a couple of inches, and placed a tin with some tuna and fruit slices inside. The white/brown rat emerged from hiding at around 1pm, and went straight to it. I pulled the string, he managed to grab a bit of the tuna and make a run for it. So I set it up again and waited. I could see him under my currant bushes, but he was in no hurry to come out. I went inside to get a coffee and came back to find the can dragged almost out of the cage, and spotted the tip of a tail disappearing back under the bush. So I set it up AGAIN and pretended to go inside the house, but kept myself hidden on my deck behind a large planter. He knows. lol.
Rat-2, Human-0.
I got tired of sitting out there like an idiot with a string in my hand, so
right now while I'm in here writing this, he's probably doling out tuna and fruit to all the other animals in the yard, enjoying a buffet. sigh.

There is an awesome website, http://www.rathelp.org/, that has been designed to help people in exactly your position.  People who have no rat experience, and have found themselves with our little friends as guests.
Here are guidelines to help you tell the difference between a wild and domesticated rat...
http://www.rathelp.org/WildVsDomesticated.html
It also gives a lot of information regarding basic care.
A few Canadian Shelters...
http://www.rathelp.org/Shelters.html#Canada
And, a listing of where you can find 'Rat Friends'.  Rat owners/lovers who have volunteered to help people like yourself out...
http://www.rathelp.org/Contacts.html
If you can, contact the website owner and ask if anyone in your area has come forward and offered to be a rat friend.
You also might want to try the ONRMCA, the owner of that forum is in Ottawa, and a lot of members are as well.  Chance are very good of you finding someone in your area to help you out.
The forum can be found here:  http://www.lilratscal.com/forum/.
They definitely sound domesticated to me, just by their colouring alone.  Hopefully things all turn out well, but they really need to be caught as you have already been advised.
Keep us posted when you can!

pictures!! 

Sounds exactly like me when I found this half frozen cat in March. 5 months later ... she's pretty much got the run of the place ... and I'm moving to a two bedroom appartment so I can put all her posts / trees / beds in it. 

If you have  photobucket or some website like that to upload and link to, you're set.  If its too complicated you can email them to me and I'll post them for you.  Just let me know and I'll PM you my email address.

Update on the little found rat...
It appears healthy, and from images I found on the web, it seems to be a female. After coming home with the bedding (unfortunately pine as I got the last message here after the fact.. it is only temporarily so I dont think it will do too much damage, considering she's been outside and exposed to god only knows)
and a bag of Nutri-Block, my DH offered to hold her while I fixed up the cage. I admit I'm too nervous to try to hold her. I guess I scanned a few too many posts online about bites and tetanus shots and the fact that she's been outside, I'm leary of possible parasites. I know, I'm probably just ignorant...

Anyway, he cuddled her and scritched her ears and she seemed relaxed and certainly didnt try to bite him, but she did attempt to get away once or twice.
As sweet as she seems to be, and as tempted as I am to keep her, I simply have to use common sense here.. I do not need to get myself into the situation of the committment that I know this little one would need. I know nothing about their health care and what problems they are prone to and I have read just enough in the past day to realize they need company of another rat and I just cant go there. lol
I will try to link two photos. I will post on the other board that seems local to me, but if anyone knows of anyone near me that would like to offer this wee one a home, please let me know. Thanks so much for all your help and I'll keep you posted in what happens.

She is sweet, isnt she?
I hadnt seen the other one since Sunday, and when I just went downstairs to make a coffee, I spotted him under my hosta eating birdseed. I took a look through my binoculars (I'm a bird-watcher), and it is clearly a male, even from that distance, lol.
This little one is curled up sleeping in the cage right beside me on my desk. I guess my presence isnt that upsetting after all. Today I'll try to make some sort of tube or hammock so she has a place to crawl into.

Hm.  If the other guys is male, you may be looking at babies....   I hope you're able to find someone to take this precious girl, and I hope you can catch her boyfriend!  And thank you for having such a kind heart to want to help.