I use card stock often to “test” colors or stroke effects. A sheet of card stock is taped beside my art work; I change it out for every new piece of art work.
Card stock matches most paper surfaces for whiteness and light bounce so it is a good sampler. You can test ink, graphite, felt tip pens on it. That process helps build a color set memory of what you have already been using, or how well a color might go with another. With felt tips, I may select a group of colors, mark them on my card stock, and file it with a copy of the artwork for future reference. At least, I should do that every time.
Make note, however: if I am using a high grade paper or special paper, I use a matching sample of that same paper for my tests, and the card stock for cleaning pen tips or polish buffing graphite points, or getting close with color choices. Then, sample test it on the piece of matching paper.
It is always wise to use a sample stroke to make sure a pen or brush isn’t going to “mis-unload” and is ready for a good set of passages, or that graphite is properly pointed and gripped. After using a sanding board or knife to shape a pencil point I always stroke the tool on card stock to refine and shape test it.
Most felt tip work is letter size paper and card stock beneath my paper protects my drawing table from felt tip bleed, also pencil-heavy hand scarring, and scratches from my x-acto knife because I’m too lazy to get up and go fetch my “self-healing” cutting mat I left on the “big table.” I keep 11”X17” card stock nearby as well for bigger work.
Card stock is: inexpensive, a ream of 81/2″X11″ (250 sheets not the 500 normal) is under $6.00.
Card stock is bright white, firm and durable.
Not good for finished artwork, but great for support work in the studio. Not archival. It is, however, stable and will last for decades in projects. Takes glue and tape well.
Card stock works well with paper craft, paper folding, and general studio support.