Hi Dimitris,
The colours in this bubble are great and they are accentuated by being against that dark background.
Now although I would not spend hours working on this photo trying to polish it up for a photography competition there are two small things that you can do to improve the photo...
Grab the Spot Healing Brush in your photo editor and get rid of the two highlights on the upper and lower face of the bubble (these are the reflections of your on camera flash).
Now, this next suggestion is a little more extreme, but I would crop the top and bottom of the picture rather hard (see example below).
Do this and I think the photo will be more interesting!
Cheers,
Anthony
Image Doctor's edited version
