I have a panel and inside a JMenuBar and I want to make this transparent, but it comes out with a gray background and I can not change it.
I put it opaque but it does not work nor does it change the background. 
I tried this code but it does not work:
UIManager.put("jMGeneral.opaque", false);
jMGeneral.setBackground(Color.red);
I would like something like this:
Where you have:
jMGeneral.setBackground(Color.red);
Change it for this:
jMGeneral.setBackground(new Color(0f,0f,0f,0f));
The Color class is used to encapsulate colors in the default RGB color space or colors in arbitrary color spaces identified by ColorSpace .
Each color has an implicit alpha value of 1.0 or an explicit one provided in the constructor. The alpha value defines the transparency of a color and can be represented by a floating value in the range of 0,0 - 1,0 or 0 - 255 .
An alpha value of 1,0 or 255 means that the color is completely opaque and an alpha value of 0 or 0,0 means that the color is completely transparent. When you build a color with an explicit alpha or you get the color / alpha components of a color, the color components are never premultiplied by the alpha component.
Take a look at the documentation that is very understandable.