Every guy owns a blazer and a pair of khaki pants. If your nuptials are leaning toward a casual, daytime affair, let your groomsmen rifle through their existing wardrobes for wedding-day attire. Be sure to stipulate and personally double-check the color of all items involved, from shoes on up. Not all men (or women, for that matter) are terribly savvy when it comes to color-coordinating, and a well-meaning groomsman might truly believe a button-up shirt to be white when, in fact, it's actually cream.
Again, this is an excellent opportunity to present them with a necktie that coordinates with the rest of your colors. If you're tempted to go ultra-casual, you could skip the tie altogether and allow them to leave their top buttons undone. Chic and practical -- particularly if the weather's going to be on the warmer side.