Rebecca asked how I made the painted background in the "why" layout, below.
The background of this layout was made on light blue cardstock, and the paint was blue and brown watercolour with a bit of gesso. If you do try this, the cardstock will probably buckle and crumple a bit because of the wet paint and water, so I try to go easy on the paint. Once the piece is dry you can gently iron it on a low setting to smooth it out as much as possible. If you want to get really painty and messy I recommend investing in a sheet of watercolour paper -- you can get it for 2 or 3 dollars for a huge sheet that will give you two 12*12 pages plus LOTS of smaller 5*7, 10*10 and 4*6 sheets depending on how you cut your scraps.
When using watercolours, make sure you mix with a good amount of water -- using them straight from the tube is not appropriate and the dried paint will be flat looking and crack. I mix up colours on an old set of saucers and just brush on randomly with an old, hard-bristled brush (I do not use my good soft watercolour brushes for these backgrounds). I just brush, pour the paint from the saucer, and drip watery droplets onto the paper until it looks right. You can allow the paper to dry thoroughly between colours or washes, or try mixing while the first wash is still wet. Sometimes I use just a little bit of gesso on the page -- dripped or brushed on randomly -- but too much or mixing it in will turn your watercolours opaque and somewhat muddy looking, so I try to keep these layers (gesso and watercolour) separate.
There really is no step by step way to explain this. It just takes some experimentation to find what works for you. Sometimes if it doesn't look right -- it just needs more. A darker colour here, a lighter colour there, something splashed over there. I do try to avoid overworking the paint too much as then the whole page just looks rather flat and muddy.
As a general rule, keep lighter colours on the bottom washes and build upwards with darker colours of paint.
Hope that is helpful :)