I just wanted to add that the rule of thumb is that the quilt is no older than the newest fabric it contains. The 1930's pastels are easy to date - stripes and solids are harder. By the 40's, synthetics began to creep into use.
In bygone days, fashions did not change as quickly as they do today, but the overall color schemes repeat in quilts, clothing, kitchen appliances and home dec.
If you jumped in a wayback machine and went to 1880, the pastels would be jarringly out of place with the somber Victorian decor. Indigoes, cheddar, and natural dyes were all that was available then, and while they fade over time, they did not spout 1930-vintage patterns.
Also, for finished quilts, smooshing your fingers to feel for cotton seeds can help reveal the age. Unginned cotton wasn't used much after Reconstruction, with the exception of some home-grown Southern vernacular pieces.