Petrol Station Crisps

I like this very much, too. I think its only fault is in the monochromatic choice of colors...and colors a little too close in value. This makes the cover not only a little monotonous but makes the title not stand out as much as it might. If you look at the cover in B&W you can see what I mean. The only black black is in the background behind the bag. Within the main art there are really no true whites nor true blacks. Everything is a little too close in value so that nothing, especially the title, really stands out.

Exactly. There are two sorts of contrast to keep in mind: VALUE, which the scale of light to dark (aka gray scale) and HUE, or color (CHROMA, or color intensity, is the third). 

Two colors can be distinctly different hues but be too close on the value scale. For instance, yellow type on a white background, or perhaps a blue on green. At the other end might be red type on a black background. The latter is slightly better but, again, red and black (or very dark grays) are very close to being the same value. (I recall an illustrator friend who had to do an illustration of a space batter for a science fiction magazine. It depicted spaceships shooting red laser beams at each other. He completely forgot that the painting would be reproduced in B&W and when it came out in print all of the laser beams---which had been rendered in red against the black background---had disappeared.)

In this color chart, there are four different HUES. The squares marked X, Y and z are each a different hue but exactly the same in VALUE and CHROMA (intensity) and are equivalent in that respect to the corresponding grey square to the left. In other words, if you were to create a cover using only the X from each of the four colors and then convert the cover to gray scale everything would disappear: the cover would be a solid gray. So the most effective cover would use colors that are as far apart on all three axes as possible: hue, value and chroma. Some of the colors are automatically very different. such as the yellow marked B and the red marked A or the blue marked D. But you can see that the red A and the green C are much closer in both value and chroma and, consequently, would have less contrast.