Everyone's gear looks the same, especially at the end of the day when it's time to head home or to camp and there is a rush to pack up. Mark your gear if you want to get it back.
It is essential that you mark your hard items. Choose a color combo you like, since you might be stuck with it for a while. Having a double color combo will make it less likely that you will have the same colors as someone else you are climbing with. Bright and contrasting colors help...but are also often chosen by others.
Electrical Tape
Super easy to use and apply
Has a tendency to fall off and leave micro litter at the crag
If marking carabineers, mark side opposite of gate to limit rope rub
Nail Polish
You don't need expensive polish
Some of the really cheap stuff rubs off easily
Easy to find in different colors and easy to apply
If used in low areas, it will last longer
If marking carabineers, mark side opposite of gate to limit rope rub
Paint Markers
Vary in quality
You can get fancy with markers, but there is no need
Like nail polish, mark low areas
If marking carabineers, mark side opposite of gate to limit rope rub
Permanent Markers
Seems like a great option, but rubs off surprisingly easily
Athletic Tape
These get dirty fast
You can find them in different colors
You can easily mark them with a permeant marker
Decorative Tape
You can find these in fancy colors or with fancy artwork
Etching
NOTE: This MAY weaken your climbing gear and make it more likely to fail
Can be done with a hand engraver
Can be done with a laser a professional finish
Professional Labels
Made to order
Labels and clear Heat Shrink
Combines easy and look of labels with durability
You can use vinyl tape or even reflective tape under heat shrink
Colored Heat Shrink
More durable than tape
Easy to apply if you have a heat gun
Limited colors...but less used method
You can use 2 or more colors if you like
instructables.com/Marking-Climbing-Carabiners-with-Reflective-Tape
Note that climbing gear gets pretty beat up and you will likely need to remark gear over time. If you use paint or nail polish in recessed areas of your gear, it can last a long long time.
There is a concern that nail polish, paint and adhesives will damage your soft times. Will it? No one seems to really know, but nail polish seems like a really bad idea.
There are special markers (Beal rope marker) that are used on ropes. These don't seem to weaken the ropes, but there is only so much you can do with black.
For ropes, you can mark the ends with tape. For longevity, clear heat shrink (Beal Rope End Kit) can be used to seal and protect your tape. Adding data such as your name, contact info and rope info may be helpful later.
Marking circular slings is a little tricker since you can get tubular heat shrink around them and because your marking or label will eventually come in contact with hardware.
People use athletic tape or labels (tagyourgear.com) to mark their gear, but there is a concern than adhesives will muck up or damage soft goods or will get caught in hardware.
If using paint or nail polish, it is possible to mark the tags on your soft goods, but you will probably cover up important info on the tags.