Is there a reason why some books on painting tell you to paint an outline first?

you have a lot of advice but I would outline it with a color in the picture that way the picture would not pick up the other color and I some times just start a painting with out an outline at all it depends on how well i know the subject

That is not a very good book. Pencil will bleed though paint over time and become visible. Unless you want visible pencil lines, this is a bad thing. The first thing to do is usually a separate sketch. Then draw an outline on your surface (canvas, gessoboard, etc.) Charcoal is preferred over graphite, as it will not bleed through. You can either wipe it off as you reach the charcoal, flick it off as you paint over it, or incorporate it into paint (it is a black pigment after all) as you desire. One of my painting instructors used blue paint. She did not worry about the specific shade, but usually used ultramarine. Her procedure was to sketch in charcoal, then shade with thin blue paint. The intent was to use the blue to show where her dark areas were. Her painting style for this was impasto. My own procedure is different from your book. I use soft charcoal to sketch the basics. Then I often make a monochrome underpainting, usually in burnt umber (sometimes scorched brown) and white. On top of this, I use a series of thin glazes and washes to add color. I use acrylic paints, so glazing is much easier than with oils. Quality of paint does not really matter. Type of paint: acrylics, oil, tempera, gouache, etc. does matter. Undercoats should be fast drying if you want to glaze. Oil paints follow a "fat over lean" rule. You can do an undercoat in acrylics or watercolor, and let it dry overnight. Then you can place glazes over the top with oil paints.

some painters, like Rubens and Titian would paint with oil tints, many layers, you can see the drawing thru the paint I have never heard of the blue line, unless this was a method to fix a charcoal line

Hm. No offense to the previous answerer, but when I got my Bachelors in Fine arts w/concentration in painting, most people in my classes actually did not use pencil first, especially in the more advanced classes. It seems like it leaves you with a stiffness. I find oils to be more about the process of painting, seeing what you will discover based on how the painting is moving along (and I'm not a "nonrepresentational" artist if that's what you're thinking). Anyway, I myself use "odorless" mineral spirits to thin down some neutral mixture, like burnt sienna and ultramarine or Prussian blue and do a sketch with that, which is easily erasable with a rag with the mineral spirits. As my paint gets thicker in the following layers, the original outline doesn't show through unless I specificallly keep them. Good luck and happy painting! p.s. Student grade is fine for experimenting, of course. I usually end up using Grumbacher or Windsor and Newton. Grumbacher red is a great alternative to pure cadmium red, by the way. Maybe it's a little bluer. (Most student ones are cadmium-barium, which using the latter as a filler.)

cobalt blue is used by some artists because it has low oil index ,fast drying and is relatively low in tinting strength .You have to thin the paint out quite a bit to do these outlines .For a portrait I wouldn't use use blue at all for this drawing ,use burnt ,raw umber ,raw sienna .You could mix a little shellac and methyl hydrate in with the outline color and this will dry and should be impervious to lifting or mixing in with the colors. cheap paints can work but when you feel more competent try the more expensive stuff by replacing them as you need them.