The totes I'm using are clear plastic with clear plastic locking lids. Maybe 6" deep and maybe 30x18" long and wide. I call them "under bed" because that's how they're marketed. I just like them because they're shallow and are easier to look through. I stack them on each other in my shop, not under the bed
When I finish a kit that has leftover parts I transfer them, along with the direction sheet, into a gallon (or quart) sized ziplock bag. I cut the end flap from the box and put that in the bag as well. I then keep the bags together in clear "under bed" totes. You know, the kind that are somewhat shallow but large width and lengthwise. I label the totes by year, or theme, of the kits. Any leftover tires and decals are the only thing I keep separate from the kits. For me, I find it easier to keep the parts like this rather than cutting them free from their sprues and trying to decide how to classify them to keep them with other like parts. It really doesn't take me too long to find kitbashing parts from these totes, but granted I only have maybe 30 or so leftover kits to go through.
Exactly my point about colors not always appearing here as they are in person. Besides, my wife always says I'm colorblind so I'm definitely NOT the one to judge color. Engine color doesn't take away from a great build which this is heading towards.