You likely make a common mistake in changing the paper color to gray. This for simulating the color of the paper the document will be printed on but it is always no color at all. You'll need to draw a gray filled frame and put it on the lowest layer in your document. Use a master page to make it easy.
Thanks, for your reply, was really helpfull.
I did what you suggested, and at the begining I could have a proper pdf with color on the background. But after tryng other stuff on the programme, same how, I got a transparency in my file, and I am back to the same point again, no color on the background, and same of the pages got the letters invisible. How can I remove the transparecy of my file?
The best way to figure out what you've done is to have one of us look at the file. You can post a link to it here publicly (and share with the world), or if the file is sensitive you can send me a link in a private message (click my name to go to my profile) or ask me to send you upload directions for my dropbox.
Thanks, for your help, but I would prefer to understand the logic of the programme, following steps towards the resolution of the problem. Any idea?
I can't guess what you did. I'd need to see the file, and then I can probably tell you what happened.
That (understanding what is wrong) is exactly what Peter is hoping to achieve with you sharing the file publicly or privately. It isn't that one of us would do your work for you. But asking and answering dozens of questions isn't efficient trying to guess what is wrong.
Far easier to examine the file and provide you with a "this is what is wrong," "this is what must have happened," and so "here is what not to do."