First we'll look at some photos of the House Finch male (don't confuse the House Finch with the English House Sparrow - similar name, different bird).
2 males at the feeder
Here's an English House Sparrow male - nothing like a House Finch
Now for the Purple Finch is all his cranberry colored glory (they are not really purple):
Juvenile Male coming into his colors. He looks purple!
Here are a House Finch pair and a Purple Finch pair. I was asked by my local Audubon chapter if they could use these 2 photos to help folks learn bird identification. I said "yes."
Now if you spot a Purple Finch or a House Finch male you should be able to tell the difference!