does anybody know the reason why some of the connection names shown in (light) gray? It can be seen alike in Connections tab and the "Select connection" drop-down.
It's a new "feature" of SQLDev v4.Prod releases (or at least I don't remember seeing it neither in v3 nor in v4.EA): I've tested it with 18.104.22.168.80 and 22.214.171.124.48.
The enviroment includes Win7(x64) with Java 1.7.0_45, and since I also use third-party drivers (to connect to MS/MyS) and they look the same (ie. some of them shown in black and others in grey) I don't think it's related to the ora client version (which is InstantClient 126.96.36.199).
This is related to the ability to define a colour for connections. The colour affects the connection name in the navigator and the connections windows have a thin border of that colour.
I also have some set to Grey although I never explicitly set the colour. You can change the colour in the connection properties screen, next to the save password check box.