Why does my crocheted dog sweater keep riding up on his chest?

Chest circumference is the number that matters most, not collar-to-tail length. For a deep-chested little dog like a dachshund, I measure the girth right behind the front legs and work to that, then handle the length separately with short rows rather than just tacking rows onto the whole body. The bunching you're getting under the front legs ussually means the armholes are too far back or too small. Before you split for the legs, add a few short rows over the chest only, not across the whole round. Work the short rows on the underside, so the belly panel gets extra fabric where the chest is deepest while the back stays the same length. That's what keeps the neck opening from getting floppy, because you're not growing the whole tube, just filling in the front. Also worth checking: the leg openings should sit forward on the body, closer to the chest than you'd think. If they're centered over the middle of the body, the whole sweater drifts back and rides up under the armpits.

On length, measure from the base of the neck to the start of the tail, but stop the body a little short of that. Sweaters that end right at the tail base tend to catch, uh, everything, and a dachshund's back end is the part that stays warm anyway.

For the variegated yarn, pooling that lands differently on each panel is just how those yarns behave when your stitch counts change between sections. If you want more even color flow, the trick is to alternate from two ends of the same skein, holding one strand from the outside and one pulled from the middle, and switching every couple of rows. That breaks up the big color blocks. Honestly though, for a dog sweater, I'd let it pool. It photographs better that way.

One last fit tip: make a swatch tube first, like a little mini version with just the neck split and two leg holes, and try it on him before committing to the full sweater. Ten minutes of frogging a test piece beats ripping out a whole finished body.

Seconding the short rows on the belly panel, that fixed the exact same problem on a sweater I made for a friend's corgi. I did mine with wrapped turns so there were no gaps at the edges, and I only needed about four extra rows over the chest before splitting. Also learned the hard way about leg placement, my first attempt had the holes way too far back and she kept stepping out of the back one. On the swatch tube idea, I'd add to make it in a cheap acrylic so you don't feel bad about frogging it twice like I did.

For my rat terrier I gave up on one-piece sweaters and just made the belly panel separately in ribbing, then stitched it to the back piece. The ribbing stretches over the deep part of his chest and snaps closed underneath, so I can fudge the fit without reworking anything. Way easier than trying to nail the shaping blind. For the color thing, I got lucky once by starting the collar from a diffrent spot in the skein than the body, and the colors happened to line up better, total accident but it worked.

I actualy doubt the technique mentioned for making the variegated yarn look more even. While changing ends can help distribute the colors, sometimes the color shift just happens because the tension isn't consistent across all panels. If you are changing your stitch count between the body and the collar, you are definitly stressing the yarn differently, which naturally changes how the dye lot releases.

Update: the short rows over the chest fixed it. I did four wrapped turns on the belly side before splitting for the legs and the bunching under his front legs is gone. Neck sits normal now too. Bonus, I worked the next one's belly panel in ribbing like suggested and the variegated pooling looks way more balanced. He wore it around the house all evening.