This color is not even in interface files, seems to be in nwindow.dll
I also thought of making the image on the server(putting the font on the image and sending it as a crest) but Mouse Over version would be black.
I know that you could make this kind of thing with Interface.u changes:
1. You need to put few textures(like 10, which will be maximum amount of those buttons) and same amount of TextBoxes in Interface.xdat file
2. In Interface.u NpcChatWnd needs to search for specific texts at the end of message(like <fancyButton value="Blabla"/> bypass="something")
3. Each text must be replaced with nothing(so it will not be shown as html), Texture made in Interface.xdat must be shown(.ShowWindow()), TextBox must the shown, TextBox text must be replaced with value.
4. Other Textures and TextBoxes must be set as invisible
5. Location of Textures must be set just after HtmlText, location of text should have fixed location(like Texture.getX() + 20)
6. Texture should react for mouseOver and mouseDown event of TextBox(so Hover and Mouse Down texture will be shown correctly)
7. Button click should run bypass
This solution has the limitation: If text is long and scroll bar shows, it might show button below whole frame. Maybe this can be fixed though.
Most likely AutoTools guy could make this for you, it is like 2 hours of work.