The difference between inks and washes is as follows.
Inks are made using a different medium than standard acrylic paint-- they use a gloss medium with a much lower pigment count, so you have a lot of control with them. They're not meant to be used straight from the pot-- diluting them about 1:1 with water is about what you're after. Inks are for making certain parts of a model shine-- things that have a slight natural sheen to them, such as metals, some hair, and laquered armor. If you wash an area of Boltgun Metal with Badab Black, you'll notice that the area has a matte finish now, rather than a shiny metallic one. If you wash the area using dilute ink, you'll find that the area retains its metallic tone. Similarly, if you wash a black haired model using Badab Black, you'll define the hairline more but will lose any natural shine if you've painted it using a drop of gloss medium (which a lot of painters tend to do with hair.) If you use an ink, you'll keep the natural shine.
Washes are essentially watered down paint. GW washes are pre-watered down to a consistency where you can use 'em straight out of the pot. They're convenient, but ultimately, you can make them by thinning whatever you've got on hand by about 8:1 water to paint. Washes are for shading without any precision work, involved, and washing tends to be one of the first techniques painters learn since it's outrageously easy. The technique described above with the white primer followed by an ink is actually washing-- you're letting the pigment, which is heavier than the water, settle in the recesses and shade the model. In simple terms, you wash a darker color over the entire area that you want to have a shade, and it naturally flows into the recesses.
With red, I've found that Baal Red is not the way to go. It's not sufficiently dark to provide much in the way of a shade unless the base color is orange or pink. Instead, I'd use Ogryn Flesh, which is a red-brown. You'll get more definition from that in the recesses, and it'll bring the orange highlights in more with the red base. If it needs more shading after that, then I'd use the Baal Red, as it'll be much more subtle.
Ultimately, there's no right or wrong way to use either product-- it's all personal preference. Do what you think looks good, and technique will follow.
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