Do dentists put silver caps on baby teeth for a special purpose (instead of white ones)?

Yes. Usually the silver caps that are placed are called as stainless steel crowns , these are placed in the back teeth the plastic white ones are placed in front teeth for baby teeth. The explanation is as these teeth will fall there is no reason to put ceramic crowns in the back or front. The back teeth take the chewing load so the stainless steel are used prefabricated usually are just fitted.
Baby Crowns. Silver caps are actually stainless steel crowns. These are placed on baby teeth because they are inexpensive and since a child will lose the baby tooth in a few years, a stainless steel crown works very well to protect the tooth in the short term. You could restore a baby tooth with an adult-type white porcelain crown, but that would cost much much more so it is rarely done.
Yes. Silver (actually stainless steel) caps for badly decayed back baby teeth are inexpensive and easy to fabricate. Dentists tend to use tooth colored plastic caps on badly decayed front baby teeth. Ceramic tooth colored caps are dramatically more costly that either the stainless steel or plastic caps. As baby teeth have a finite lifespan, silver caps are a better economic choice.