Rainbow is a Minecraft server mod.
Website: http://www.project-rainbow.org Builds: http://ci.codecrafter47.dyndns.eu/job/Rainbow/
This project uses maven. The project can be build by running the following command:
mvn clean install
After the command has run the final rainbow server jar can be found in rainbow/target/Rainbow-<VERSION>.jar.
I recommend using IntelliJ. It has good maven support and its integrated decompiler is very helpful for projects of this kind.
Before trying to import the project into the IDE I recommend building it from the command line the first time.