Here I've turned the opacity on my outline down so low I can barely see it.
Then, using a big, very soft, very low opacity brush, I paint just around the inside in the same light colour I've used on the rat's highlights.
I've used a photoshop mask (works like masking fluid in watercolours) to help me not go over the edges here, since I'm using such a big soft brush, but you could easily make a brush with a hard edge on one side -I just prefer this way. ;-)
Using a low to medium opacity brush again, I'm going over parts of my bottle in a blue-grey colour.
Again, this brush is soft, but not AS soft - we're looking at a hardness of about 20%.
I continue to do the same, building it up slowly. I'm looking at various photos of bottles as reference to see where light hits.
You can see I'm building it up mostly on the side of the bottle closest to my light source (which is in the upper left corner), but also adding a small amount to the lower right-hand side of the bottle - light does, after all, shine through a clear glass bottle.
Now I start adding more serious highlights.
I switch to a light peach colour, and up the opacity and hardness on my brush, then paint in some blobs and dashes for highlights, trying to keep the brushwork loose.
Keeping in mind the shape of the bottle, I add in a couple of random streaks - they'll help define the shape, and help make my bottle look a tad grubby. The loose brushwork helps with this - if I over-worked it to a perfectly smooth gradient, I'd lose the "life" in the bottle. This way, it looks more textured, and that's what I want.
I up the saturation of the colour on my brush, making it less pastel and more vibrant, and add more highlights.
It's important for me here to build up layers, so I keep a reasonably low opacity and just keep building it up with loose brushstrokes. The temptation here really is to over-blend and over-smooth, but it'll ruin things in the end.
On to the next step!!