In the 1930s, the two-tone colours images were being used a lot and were really coming of age. The images associated with automotive influences such as posters. Automotive advertising was said to be another new industry of the machine age, of the 1920's. This age naturally incorporated the new design esthetic of the 1920s and 1930s.
Images with single cars, slanted on its side with a long windy road were popular in a lot of the thumnails and posters from this time. As well as the dull two or three colour which were used with changing gradient to create different tones. There were often stripes of colour, which designers used to emphasise the speed of the automobile. Mainly blues and greys were used as dominating colours to achieve a sketch like look, with subtle patches of brighter colours such as red, orange or yellow. Most the type used on these particular posters were thick and bold to get the readers attention and to get the point across. Designers in these days had simple and sometimes quite dull colours with the main focus eg the car in the middle and on a slight slant.
These thumbnails were interesting to look although I don’t think they would serve their purpose in this day and age.
The link i got this info from was:
http://www.travelbrochuregraphics.com/Top_Level_Pages/Automotive/Automotive_Page_3.htm