I'll skip the whole "don't stress, rain is good luck, it'll be fine shpeil" and I want to jump straight to the helpful tips! If it does happen to rain on your wedding day and I am your photographer, you're in luck! That's because I LOVE rain and will help keep everyone in a good mood!
Do a First Look! *THE MOST IMPORTANT TIP
Maybe you already planned on one, but if you did not, rain is a great reason to seriously consider otherwise! If it does happen to rain at your wedding, you now have two windows of time to photograph, before the ceremony and during the cocktail hour where hopefully the rain slows down or even stops. It's better to plan for that in case the rain gets worse. The later it gets, the darker it gets also! It's better to take photos twice and have more of them, then risk it after the ceremony when the rain could turn torrential!
Prepare ahead of time
Have a back up location if your ceremony/reception is outdoors…ALWAYS (especially if you wedding is in the summer in Colorado. You never know when it will rain, but you can always count on those summer evening rain showers)
Buy Umbrellas for photos and rain boots
I ALWAYS have spare umbrellas for the bride and groom in my car. It's always ideal to have natural light for photos, rather than indoor. Overcast rainy weather is normally great lighting too! Have some umbrellas you can jump under so you can still get those gorgeous outdoor portraits. The rain boots are great to wear when walking from one location to another to keep you wedding shoes clean. It also make for a really adorable photo!
Scope out locations
If you don't want umbrellas in all your outdoor photos, scout out the venue ahead of time for an outdoor location that provides a covering from the rain. A gazebo, a really thick tree, or an overhang. All of these can provide shelter and you don't have to sacrifice gorgeous outdoor photos.
Have your makeup kit close by!
This way you can do any needed touch ups when out taking photos! And go WATERPROOF! I recommend this, even if it's not supposed to rain. This will keep you make up in place and looking fresh all day!
Be Flexible with time!
If it stops raining during your reception, be willing to sneak out and take a few photos. Likely the sunset will be amazing also! It's easy to let the DJ or MC know that you're going to push speeches or a few things back while you take photos.
Have fun and trust your wedding photographer and other vendors to have the experience to handle some rain!