You may think there’s not much difference between watercolor paper, other than some being of higher quality, but that not the case. There are different textures, weights, and colors. That’s right, colors. You thought it was only white, but there’s traditional, bright white, extra white, and absolute white, and depending on the company it can a noticeable or not so noticeable difference. The color of the paper will effect the look of your painting. A cream tone paper will give a brown or muddy look to your painting. While a paper with a blue tint to it will make your yellows look slightly green. So depending on what you’re trying to achieve, you may choose different papers at different times.
Many people are familiar with the different textures; rough, hot press, and cold press, but they’re not as familiar with what they’re just suited for. Rough paper is the most grainy, and generally not good for fine detail, but since it is so smooth many people, especially beginners, have problems with paint sliding around. Cold press is the most popular because it allows for the fine detail, but also has some texture to it. Also with all of that, there’s a “right” and “wrong” side of the paper. The “right” side is more textured than the other, so the “wrong” side would allow more fine detail.
Then there’s also weights of papers; 90lbs, 140lbs, 260lbs, and 300lbs. Thinner paper needs to be stretched so that it doesn’t buckle when painting. The weight you’ll need in order to not stretch it will be different depending on how you paint, but for many people, they need at least 260lbs. Stretching a piece of paper isn’t the end of the world, but you may need a heavier weight in order for it to take the amount of layers you want, and if you tend to abuse your papers, you definitely will want a heavy paper.
So now that you’ve figured out the color, texture, and weight, now it’s time to buy the paper, but do you buy a roll, individual sheets, a pad, or a block? These are the questions you need to ask. How much paper do you use? How do you paint and what sizes do you use? How much are you willing to spend? Rolls are nice because you can cut the paper to whatever size you want. So if you’re using a lot of various sizes this might be a better option for you than buying tons of different pads. Sometimes you get get deals on bulk individual sheets verses buying pads or blocks. This is usually only an option for the artist grade paper. Pads are the most commonly used and sold just because they’re convenient and cheap. They store easy and you can tear off the page or keep it on. Blocks are more expensive, but they have the convenience of not having to be stretched since they are attached on all sides. If that’s something important to you, and you’re willing to pay for it, blocks could be an option for you. However you can’t pull it off until it’s dry, so you can only work on one painting at a time unless you get more than one.
It all depends on your personal preference and style of painting as what paper you use. I still wouldn’t recommend using anything less than acid-free. After that, you can use a matte spray varnish if you wish. If you’re just starting watercolor, finding a cold press 140lbs paper should work out nicely for you. After that you can try experimenting to see if there’s something you like better.