In this case, "even" is just a short, poetic word for "evening." (If Shelley spelled out the whole word, it would mess up the meter of the line.)
So that part's easy, but what about the way the evening "melts?" For us, it doesn't call up a specific image, so much as the feeling of the purple sky flowing around the flying bird. Trippy, dudes.
Don't miss the alliteration, too: "pale purple." A pretty perfect pair, we think (okay, that's probably why Shelley writes the poems, not us).
Like a star of Heaven, In the broad day-light
This is the setup for a simile that ends on line 20 (notice that it starts with "like"). It might not make sense until you read the whole thing, but the speaker is describing something here that is present but invisible, the way a star might be during the day. It's still glowing; you just can't see it.
Thou art unseen, but yet I hear thy shrill delight,
Now the speaker finishes the simile, comparing the invisible bird to a star that is hidden by the daylight. It's important to notice that this poem isn't really about what you see with your eyes. It's more about what you hear and imagine.
That "shrill delight" thing is interesting, too. We associate the word shrill with unpleasant noises. We're not exactly saying that the speaker doesn't like the sound of the skylark's song, but there is a feeling that this whole thing is almost a little too much, a little out of control.