Some additional remarks on exercise 10.
1. b)
The problem was solved by first drawing a line through c3 and c and
finding its intersection with the line from c1 to c2. From this p3
could be found. (Then using the ratio R12=p1/p2 the problem was
solvable.) There was some discussion then if one could have started
from some other point, say c1 and draw a line through c and thus
finding p1. Then one would do the same for c2 and find p2. This is
indeed possible as well, which can be intuitively understood...
The percentage p3 is however _not_ proportional to the line from c3 to
c, but instead to the line c to c0. Because the larger c3-c is, the
further away the point c is from c3, an the smaller the percentage
p3. (This fact can also be seen from exercise 1.a), where the
percentage of p1 is proportional to the distance c-c2, not c-c1.) In
the exercise session I misleadingly highlighted the c3-c line when
talking about the percentage p3.
Looking at the image in the model answer, the line c-c0 is shorter,
about 1/3, or 33%, which seems more plausible. Proving that p1, p2 and
p3 calculated in this way indeed sum up to 100% in the general case is
however a bit complicated :)
3.
The CMY colour model use the colours cyan magenta and yellow as it
base colours. These are often called the secondary colors of light
(since they are at the opposite ends of the RGB colour cube), but also
the primary colours of pigments. Cyan for example is the opposite of
red. This means that if we shine a white light (which is composed of
equal amounts of red, green and blue) on a cyan surface it will absorb
only the red part of that light and reflect the others. Similary
magenta is the colour that absorbs green and reflect others, yellow
absorbs blue. This is why these colours are more natural to use when
for example printing on a white paper area.
The CMYK colour space is often used in real printing applications. The
K stands for "key", but is in practice the pure black colour as
discussed in the exercise session. See e.g.
http://en.wikipedia.org/wiki/CMYK