Well there is the Quick Color Picker for Notepad++
You double-click on the hex color and a color-picker pops up as well as a preview. Another cool thing is that it provides a few more colors that match the "theme" if you will.
Another one is the html color plugin for notepad++
You select the color code and click a button to show the preview. From the preview you can change the color by a few clicks. It can also be used to insert new color codes.
That's all I have for you right now in the way of that type of plugin. There is also always google if you need to try to find more but as far as I know, those are the main 2 that do the trick.
I haven't personally used them yet, but since you brought it up, I'm thinking of giving both try myself perhaps sometime. I usually just google it and also have a color picking extension in my firefox called "ColorZilla" and that extension is awesome.
Have an awesome day man!