There is some talk of piping water all the way from the Mississippi River to the Colorado. Expensive for sure.
I've come up with my own idea for putting more water in the Colorado. One of the big users of Colorado River water is the Imperial Valley in California. That's a big agricultural area right on the Mexican Border south of the Salton Sea. I think it's actually a bit below sea level.
Maybe they could build a desalinization plant on the Gulf of California, in Mexico, to provide water to that area. It might even be able to use a mostly gravity feed pipe or canal down to the Imperial Valley from Gulf of California.
Desalinization could provide fresh water for both the Imperial Valley and parts of Mexico. Those areas would no longer have to be so dependent on Colorado River water.
I know that the idea of desalizazation may not pencil out economically for agriculture. Desalinization is an expensive process. Too expensive to provide water cheap and abundant enough for agriculture. Farming needs a cheap source of water, unless it's hydroponic; like indoor agriculture; Another topic. That might sink my "Gulf of California / Imperial Valley" idea, but I still present that idea here, at least.
The process of desalinization is already starting to be used by California's coastal cities; such as San Diego. Urban areas don't need as high a volume of water as open field agriculture. The concept of desalinization is starting to pencil out for urban water usage.
Bringing water all the way from the Mississippi would be expensive as well.