$ GET -e -d http://color.com/ | grep Server
  Server: Jetty(7.3.0.v20110203)

Jetty server, so presumably Java.

  $ host 50.17.223.164
  164.223.17.50.in-addr.arpa domain name pointer
  ec2-50-17-223-164.compute-1.amazonaws.com.

Hosted on EC2.

  $ host -t ns color.com
  color.com name server ns2.p18.dynect.net.
  color.com name server ns3.p18.dynect.net.
  color.com name server ns1.p18.dynect.net.
  color.com name server ns4.p18.dynect.net.

With Dyn (DynDNS) handling DNS.

That could also represent JRuby, Jython, Scala, or any other language that runs in the JVM.

The iPhone client is written in Objective C.

The Android client is written in Java.

The webserver appears to be written against the JVM, so it could either be plain Java, JRuby, Jython, Scala, or any other language that runs in the JVM.

DJ Patil however, was LinkedIn's Chief Scientist, and LinkedIn is a heavy Java/J2EE shop, so my guess is that it's vanilla J2EE.

One clue is that a 404 page (e.g. http://color.com/asdf) says the webserver is "powered by Jetty": http://jetty.codehaus.org/jetty/

Jetty "provides an HTTP server, HTTP client, and javax.servlet container". Here is the wikipedia page for Java Servlet: http://en.wikipedia.org/wiki/Java_Servlet So the answer seems to be Java.

if its java, wonder why they chose it?

DJ Patil is LinkedIn's Chief Scientist, and they're a big J2EE shop.

What makes you think there's any server-side code running at all? It looks quite straightforward to me.