Hullo N.G.
First off, it certainly can be done. Amazing, really: overlays in a cheapie like Pages. But they're there, and their possibilities are all but inexhaustible.
I won't give a detailed scenario - you'll discover more by exploring.
You can manipulate the opacity of your main text flow (what you normally type into Pages, without a text box or shape) through the colours menu. However this makes overlays difficult. Best to put what you want into an object first: if it's text, then a text box is all you need. Set that to as much of the page as you want to fill with background (you have only to click on it, at this stage, to find handles to size and position it, or double click to put your cursor inside it to enter text. Once you've used it and moved it elsewhere, you will need to click outside it - in the margin of your layout, and then "select all" from the edit menu; or keyboard shortcut command / a. This will bring up the handles, again, and with them the option of double clicking to access the text in the object. Harder to describe than it is to do.
If you want the text to cover the whole page, bled out to the margins, then leave yourself a little space to deal with it from, and use the marginless printing options in your print dialogue box (if your printer supports this) to leave them out in production.
Use the object inspector to make the object "fixed on page" and then reduce its opacity to less than 100% depending how pale you want it. Now turn to the wrap inspector, and deselect "object causes wrap". When you're satisfied, use "send object backward" from the arrange menu (a keyboard shortcut is given there) and you're ready to overlay it with either normal text; or for somewhat more flexibility, with another text box or shape into which you can put either text or text or images respectively.
Provided you're only using two layers, using normal text entry for the second may be all you really need. In more complex layouts, however, various levels can interact, and it's probably better to have separate objects for everything you want to overlay or to overlay with something else.
Remember the "undo" feature in the edit menu if you get yourself into strife; but once you've got what you want, explore the options in format / advanced to manage and capture pages. Do the same with paragraph and character styles, and you'll soon enough find you can make your own templates to save the layout work you've done for use in future documents.
Pages is a can-do program. Best of all, you can see what you're doing with it.
Cheers.
iBook G4 Mac OS X (10.4.6)