I usually do a Google Image search of said item + the exact color name and see as MANY different versions of the color that I can. That way, I'll know if the range is acceptable for me, or not -- because shopping online, you need to be in range, at the very least. The very best measure of the color isn't the professional white/background floating photos, it's the "for sale on ebay" photos you see in all sorts of weird light. They let you see just how good or bad they can look.
I just got finished doing a very detailed color search for some boots in my radar. I couldn't find enough photos of said boot to ensure they weren't going to be a tragic mistake ( they were a "burnished" boot...I need to know what they mean by BURNISHED!). I couldn't find enough evidence to assure myself they would be fine, so I didn't purchase.
I will say though, I have that same bag in Kelly green. I purchased it this spring, and I LOVE it. It's a great size with lots of inside organization -- though when I bought it, the entire line of them were skittle-colored though. The green was the least bright one I could find. I can imagine how bright yours might be, if it's in the same range as my green.
Below is the range I found when I searched for your bag. Some I'm sure are VERY off from what you have, some are probably closer. Discount anything in the wrong material (leather, in this case).
I searched "Dooney Aegean Blue"