You can use as much styles in a models as you want. But only one style
can be displayed at one time.
Go to the styles window, to the lin style tab. On the bottom of the
window, you can choose if the colour of the lines should be monotone
black or according to the axes (very helpful for modeling) or
according to the material. Choose the last one and use the paint
bucket to color the lines.
I would also recommend to make the contour lines into one group and
the plot lines into another. This way, they do not stick together.