How do you store & organize CARDSTOCK?

My 12x12 cardstock is sorted by color and stored on bookshelves in paper holders similar to THIS.

As for the 8.5x11 cardstock, a selection of it sits on my scrap desk in one of these trays. (This one is from Walmart; but mine from Costco is very similar.) The rest of my 8.5x11 CS is on bookshelves in various magazine holders. They're not all the same - some are cardboard that I bought really cheap from Staples & various other places, and others are nicer more sturdy wooden ones from Ikea.

The paper trays on my desktop were a wonderful find at Costco several years ago - I love having an assortment of all the colors within arms reach of both my scrap desk and my computer desk (where my Cameo also sits) so it's accessible for die cutting, card-making, punching, or other types of cut page embellishments (page borders, banners, etc.). When I'm scrapping, I've typically already pulled the 12x12 paper (both PP & CS) that I'll be using for the project and have it sitting on the "L" arm (free-standing small shelf unit on wheels) of my scrap desk, either stacked on a shelf or in an open Iris case on top.

I recently switched my storage a bit.

My paper is still in 13 x 13 plastic envelopes.
Includes solids and patterned.

The envelopes are in bins (my new way of holding all the 13 x 13 envelopes). >> www.hobbylobby.com/Spring-Shop/Storage-Planters/Baskets/Rectangle-Ribbed-Container/p/81016548

By color. 
Multi-color is by style (striped, polka dots, water color, grunge, floral, etc...).
Certain paper is by brand (Basic Grey, Daisy D).
Themed paper is with that themes other stuff (stickers, embellishments, alphabets, etc...) in 13 x 13 envelope (Christmas, Birthday, travel, etc..).

I have a plastic file box, with hanging folders, and whole cardstock sheets go in there sorted by color. Scraps are stored in big Ziploc bags by color.

It's just a Sterilite file box that you buy at any office supply store, with those army green colored folders, and yes the cardstock is 12 by 12. The box is just a tiny bit wider than the paper, but it does fit.   The paper sticks out the top of the box.

I keep all my cardstock in a small drawer unit on top of my desk. All my cardstock fits in one drawer and is sorted in rainbow order.

I use vertical storage bins from the container store.
I use one medium one for red/pink/orange/yellow (warm colors),
one small bin for greens,
one small bin for blues/purples,
one medium for neutrals (black, brown, gray, etc.),
one small for whites,
one small bin for watercolor cardstock.
The six bins all fit in two cubes of a target cube thing.
Textures and brands are mixed together but I do try to keep cardstock that I have more than one of together.

Sorted by color in stacking, oversized paper trays. (Flat)

solid cardstock by color
patterned cardstock by manufacturer

they are in bins where i can easily leaf through them.

I have one of these as well (if there is a wire drawer at the bottom yours is probably from the container store). Mine are by color. 1 file for full 12x12 sheets and one for very large scraps and 8x11.

Smaller scraps go in the large Sterlite 3-drawer bins by color, along with scraps from my decorative papers. Super easy to pull a drawer out and put on my desk so I can grab what I need.

If I buy cardstock to go with a specific collection, it is stored in the 13 x 13 clear envelopes with the rest of the items in the collection. Or to go with a specific layout, it goes in the page kits with everything else.

By color- White, Black, then ROYGBIV, then neutrals. I try to keep like papers side by side but I don't bother arranging my reds, for example, in any particular order.
I have all different textures put together. I also have Bo Bunny paper (dot, ledger, etc.) stored within the color.
Most of my regular cardstock is linen texture. I have some orange peel and other textures but hardly any smooth unless it is kraft. I don't buy smooth but I have some that came in kits.

I have the majority of it stored in a big Rubbermaid type container that is rectangular, so it fits the paper. I also have a couple more portable Iris/Art Bin type carrying cases that I use to transport my cardstock to crops, etc.

In my big container I have a thin Totally Tiffany paper holder (I can't think how to describe it- like a thin magazine holder?) in front of each color. I put all of my large scraps in there. In theory I should look in the container before I pick a full piece. Sometimes I do, sometimes I forget.

Why is it that no matter how many colors of paper I have, I never have the right color?

So organized! Have I ever mentioned that I want to be Nancy when I grow up? 

Slight highjack: What are the pocket cards on the bottom shelf stored in?

I think if you tend to buy coordinating cardstock for PP or inks etc... then by brand makes a ton of sense. Or if you need to label the products you use (like on a blog or youtube or if you're published...).

For me - I'm going to flip through all my yellows to find the right one so I store all my yellows together and when I start to run low/out or I realise that I don't have any lemon yellow for instance, then I'll pick some up next time I'm shopping and it'll be whatever brand is available (am I at HL or M or J or...) so brand is less of a factor for me. I do have preferences - I like loose cardstock (which is probably Brazzil), I like the textured pads at M's or the Colorbok from WM, I prefer M's bulk packs to HL's - HL's doesn't cut as nicely ime - but generally it's more a case of buying what's available where I am - I have HL in town but M's and J's are about an hour or so and we're only there once a month-ish and I try hard not to go to WM for just one thing.