There are a number of words for which there is no rhyming word in the English language. Here's the technical "linguist's" reason: Because English is no longer an inflected language, there are fewer words which would have rhyming inflected endings, for example.
Whether you know what that means or not is neither here nor there, however. Suffice it to say, there are no English words that form a "true rhyme" with either "orange" or "purple." There are other words as well. "Month" is another one, for example.
Because English is what is known as a "rhyme-poor" language, we allow what's called "slant rhyme" and "sight rhyme," and that gives poets a little more room when they search for a rhyme.
Example of sight rhyme: My living room's hearth is the coziest place on earth. The two words LOOK like they should rhyme, but they don't. So that's an example of "sight rhyme."
Example of slant rhyme: His brow furrowed as the flag unfurled. The words are not a true rhyme but if you look at it squint-eyed and listen squint-eared, then they're close enough to count. ;-) You can also make a case that "brow" and "furrowed" are slant rhymes.
