Why does my cardstock show a white line on every folded edge?

Most of the white you're seeing comes from the scoring creasing the surface coating and exposing the fiber core, so part of the fix is mechanical rather than coloring. Score with a harder tip than you think you need, then do the actual fold over a rounded edge instead of a hard table corner. I use a bone folder against the back of a spoon handle, folding in stages, and that alone cuts the white line way down because the paper bends instead of cracking. Pressing harder on the score actually makes it worse at 250gsm, it just cracks deeper.

For the coloring side, markers work but they're hard to feather into the print and they can bleed onto faces on tight bends. What I've had better luck with is soft pastel or a chalk pencil, picked up on a small finger sponge dauber and rubbed along the raw edge before folding. It's dry, so glue doesn't react with it, and you can blend it into the printed green so the seam stops reading as a line. If I can't match the color I go with a dark grey or brown rather than black, black looks like a hard outline from a meter away. Some people use ink pads the same way, just dragging the dauber across the pad and then the edge. Do it before folding whenever you can. The edge spreads and stretches when you make the bend, so touching up after assembly usally means working around seams and you can see where you stopped and started.

And yes, the brand matters more than people admit. Some card is white pulp all the way thru, some is dyed through the sheet, and a few are laid up with a colored core. If you can find a card with a dyed core in a mid tone it mostly solves the problem on its own, since the exposed edge is allready colored and you're only blending, not covering. On sanding, I'd skip it on the small parts like your optic cutouts. A fine foam pad can knock a fold edge over on big flat pieces, but on small curves it rounds the corner off and you lose the crisp line you paid for by using the heavy stock.

I do folded gift boxes out of heavy colored card and the white edge problem has been bugging me for ages, never knew pastel could fix it so thanks for spelling that out. I've just been living with it thinking it was unavoidable. One thing I'm unsure about though, does the chalk stay put after you fold? I worry it would rub off onto my fingers or smear onto the printed face while I'm handling the piece. Also I'm guessing regular soft chalk pastel sticks work the same as the pencil kind, or is one easier to control on a tiny dauber? Still figuring all this stuff out

I ran into this exact thing on a little castle model I made out of thick card, white cracks all down the roof folds. One thing I figured out by accident was to fold the piece so the bend cracks on the side nobody sees, meaning I flip the part and curl it over my thumb slowly instead of over a table edge. The white line ends up on the back and the front face stays clean on most pieces. That stick thing people use for flattening folds, the smooth plastic or bone one, does any hard rounded object work the same or does the material acutally matter?

Back before anyone had fancy dyed-core card we just went at it with a soft colored pencil, the cheap waxy kind from the kids' art shelf. Run the side of the lead along the score line hard enough to leave a film, then burnish it in with your thumbnail before the fold. It acts a bit like wax on a saw cut, fills the fibers so the white doesn't come back when the edge stretches. On green camo I'd grab the darkest olive pencil I had and it disappeared under my living room lights. Downside is the pencil can skip on short folds, so on something tiny like an optic ring I'd still fall back to a paint touch-up with the tip of a brush after it's glued.