I created a symbol based from a symbol I searched from the internet. It was a Celtic symbol. I revised it a little bit and put some colors in it. Considering that the symbol is already very old and I created my version of it from scratch, am I still subject to copyright laws?
Yes, copyright applies
First, AFAIK, the Celts did not have access to the internet or, if they did, their access speed was horrendously slow. Therefore what you found on the internet was not created by the Celts - it was a photograph or a drawing created by somebody else and probably is still under copyright (exceptions apply).
Let's assume that what you saw on the internet is a faithful representation of something that was actually created by the Celts before copyright existed and therefore the original object is public domain. If the thing you saw on the internet was, say, a photograph of a piece of Celtic jewelry and that jewelry had the symbol on it then the symbol would be public domain but the photograph isn't if it's composition required creativity.
Whether you have copyright in the image you created depends on if you met the threshold of creativity in your revisions and colors to create an artistic work.