My go to size colorado is #5 and #6. Go to for double willows is size 4 i believe.
Ive had success with double willows all the way up to size 6, singles up to 8, and colorados all the way up to size 8. Some colorados have more of a cupped bend in the round area of the blade. This was intended to achieve better action at slower speeds, but you can pull them at faster speeds as well, just be sure to use a quality ball bearing snap swivel. That goes for any type of harness fishing. Willows were made for faster speeds, but you can run them at slower speeds as well, just upsize the blades to get the same action and depth if you feel necessary. For any type of spinners, the bigger the blade or blades, the more resistance or pull you will have.. This will means a #5 colorado will run slightly deeper than a #8 would on the same lead and speed. Bigger size blades are meant to be pulled at slow speeds imo.
I use 30lb coated nylon wire for my double willows with folded clevis's. I dont use quick change for them because nothing is more aggravating than losing one blade on a double willow and not having a replacement. You can also use 30 or 35lb flourocarbon with folded clevises and hold up good. Thats what silver streak uses for their pre-tied double willows.
I use quick change on colorados, because that what im pulling 75% of the time. I need to be able to quickly change blades if needed. If i lose one, its no big deal, have plenty of back ups.
Hatchets are fun. I run sizes 5-8. These are also made for slightly faster speeds than colorados but can be pulled at any speed. If you slow down, just up size the blade for same action if you want. These put off a thump sound that big fish cant resist. Before a walleye even sees or smells a bait, he can feel it. He can tell how big it is, how fast its moving, and which direction it is moving in. Thanks to the lateral line. Hatchets trick big fish into thinking the bait is actually bigger than it really is, because of the unique vibration they put off. Still, they catch fish of all sizes, but are known for big fish.
Dont get too caught up in the different colors. Water is a color filter, and most colors are just a shade of grey to fish after 20 foot or so if remember correctly. Even less on a cloudy day, or muddy water. Patterns vs. solid color on the other hand can be a big trigger factor tho. Lets say you have pink blade with a black dot on it. Even tho the fish cant distinguish the colors we see, he can still see the pink as a light shade of gray, and the black dot as a dark shade of grey.
Time to get back to work. Hope this helped.