When I moved out to California in 1980 I was lucky enough to live close to three very unusual fiber art stores. They all carried unusual old threads. Thread junkie that I am, I would, of course, buy them.
The results were not always good. Cotton wouldn’t be strong enough to use, wool would shred before I could even use it, and silks were so thin they would get hopelessly tangled.
There are perils and risks when you use older threads. In addition to these colors may be different. This is because fibers get dull with dirt and age. They lighten if exposed to sunlight. And, finally changing environmental laws mean some dye formulas have had to change.
Mary Corbet’s Needle ‘n Thread bog has a great in-depth post about older embroidery threads.
Neither she nor I would say avoid them, But this is one case where you should be especially careful.