The Tetrahedron Coloring Problem
Solution: #3 is different from the others. The others all fold into the same tetrahedron.
There are exactly two ways to color a tetrahedron using 4 different colors, and the two colorings differ by a reflection.
Looking at the nets below, we can see that #1 and #5 are really the same tetrahedron because the nets differ only by a rotation. So the answer must be either 2, 3, or 4.
You can also take one of the nets and move the faces in a careful way so that the edges you reconnect are also edges that meet up in the folded 3D figure. That way you have an equivalent net. In the diagram below, I started with net #2, and moved two faces to end up with an equivalent net that is a rotation of #5. Now I can conclude that the "different" one must be either #3 or #4.