Rainbow Duct Tape is an item that can be used to help complete various buildings. If you have enough you can use it to complete a structure. The Rainbow Part Center drops it and allows you to carry more Rainbow Duct Tape in your inventory. The inventory maximum is 5, with a Level 5 Center, unless you pay City Cash to buy a bundle.
If you have enough Rainbow Tape, you'll see the blue button showing the Cash amount needed turn to a green button that says Finish. This will only happen on structure where you have enough Tape to finish the project.