Is there any official or widely accepted Hex Color Code for the color symbolizing Esperanto?
The Esperanto Wikipedia article about the flag happens to have a section about this.
There is no official color, and various organizations and people have used various shades of green for the flag and other Esperanto symbols, including #009900 (in the website Flags of the World as well as the SVG file found in Wikimedia Commons), #007D13 (TEJO), #37A93A (Google), #2C693F, etc. The flag itself doesn't seem to have been made official by UEA.
There was even a debate in the Esperanto Wikipedia about this, where it is made clear that no official color exist. So, basically, you can use any shade of dark green you want.