Once upon a time, before Mac OS X, : was the directory separator instead of /. Apparently OS X 10.7 is still trying to fix up programs like that. I don’t know how you can fix this, if you really need the : to be there. I’d omit it :-).
EDIT: After a bit more searching this USENIX paper describes what is going on. The rule they use apparently is this:
Another obvious problem is the different path separators between HFS+ (colon, ‘:’) and UFS
(slash, “https://stackoverflow.com/”). This also means that HFS+ file names may contain the slash character and not
colons, while the opposite is true for UFS file names. This was easy to address, though it
involves transforming strings back and forth. The HFS+ implementation in the kernel’s VFS
layer converts colon to slash and vice versa when reading from and writing to the on-disk
format. So on disk the separator is a colon, but at the VFS layer (and therefore anything
above it and the kernel, such as libc) it’s a slash. However, the traditional Mac OS
toolkits expect colons, so above the BSD layer, the core Carbon toolkit does yet another
translation. The result is that Carbon applications see colons, and everyone else sees
slashes. This can create a user-visible schizophrenia in the rare cases of file names
containing colon characters, which appear to Carbon applications as slash characters, but
to BSD programs and Cocoa applications as colons.