Transparency for color
 

How to add transparency slider for existing color pls?

This is a bit of a two step thing. Assuming you want it in the settings menu, you can create a slider with LibAddonMenu. Check out the github page for it, or another addon for examples.

Once you have a slider with LAM, have it set a saved variable to whatever value is chosen, and then you can use SetAlpha(a) or SetColor(r,g,b,a) to change the transparency (alpha is how transparent it is)