Yep, it's a tough question, and depends on what your personal style is. My first was a 05 BG Pink City, and I immediately became a lover of the vibrant Bal colors. I have sold any dull or darker bags I had because I am always reaching for my vibrant bags to use, and leaving the darker ones in the closet. I could never see myself buying a white, black, or brown Balenciaga bag, yet those 3 colors were the only colors I used to buy and use before I discovered LV and then Balenciaga.
Then there is the question on whether you want to buy a current bag, so you can purchase a new one from a store, or would you like to go for a bag from a past season off of ebay? I think Grape would be a good new color to go for, because it is a pretty color, yet not too vibrant so you should have a good chance it will go with a lot of things. Although, I must say my bags go with many more things than one would think by looking at them.
Lastly, I would suggest making sure whatever your first bag is, try to get a color that will go with many things in your wardrobe. When you take the plunge and get your new baby home you will want to use it all the time at first, so it would suck if you had to leave it home because it isn't going with whatever you're wearing.
Take a look around the Bal Forum like the library section, collection photos, and modeling pictures. That will definitely help you pin down the ones you like. Don't forget to come back and post pictures of whatever you buy, and good luck finding the perfect bag!

Below is some pics of my bags. The Pink City in the upper right corner of the group photo is no longer with me, but everything is what I have today. Oh, and my Violet City with regular hardware arrives tomorrow, sol I will let you know what I think of it firsthand...