ok, first of all i can only advise you again to get another textbook, because not only does your textbook seem to be unreliable, it also uses grammar in early lessons obviously without explaining it. if you don't trust me on this let me tell you that i've just majored in urdu which has just the same grammar as hindi.
to your problem:
"rang" simply means "colour".
"ka" (masc), "ki" (fem and plural fem), "ke" (plur masc) are particles constructing the genetiv case. you could roughly translate them as "of", mostly you don't translate them at all but just let them compose nouns.
The sentence "purs kale rang ka hai" literary translates to "the purse black colour of is". See the similarity to a similar english construction?: "the purse is of black colour".
Now what's the problem here? I'll explain it step by step (which actually your textbook should be doing):
1. We have an inverted sentence here, which is necessary because the sentence is supposed to express the quality of colour of the object. If just the fact, that there is an object with that colour is expressed you could see the following sentences:
"(yah ek) kale rang ka purs hai", "(yah ek) kale rang ka batua hai", "(yah ek) lal rang ka ghar hai"
("it is a purs of black colour", "it is a wallet of black colour", "it is a Haus of red colour")
There is no whatsoever reason for a sentence like "purs safed rang ki hai". "rang" is male, "purse" in my opinion is also male and the genitive particle can't agree with safed because safed is no noun at all but an adjective. it simply makes no sense (native speakers please correct me if i'm wrong).