What's wrong with this LL(1) grammar?
Your original grammar is ambiguous:

L → L : L
L → id R


Suppose we have id : id : id. (For simplicity, I'm just letting R derive the empty string, but any valid derivation of the Rs would end up with the same ambiguity.) That needs to be derived from S → L → L1 : L2, but from there, either L1 or L2 can derive L : L. In effect, the two cases correspond to : associating to the left or associating to the right, but with longer sequences of :-separated id R clauses, the number of possible parses increases exponentially.

Your rewrite does not eliminate this ambiguity, it just makes it harder to see.

In short, you need to decide how : associates. It probably makes most sense for it to associate to the left, but an LL grammar really only handles right-association. (That's not a huge problem since you can reassociate the AST easily enough, but with an LR parser generator you wouldn't have to do any of this.) If we rewrite L as right-associative, we automatically eliminate the left-recursion problem:

S  → L
L  → L'
L  → L' : L
L' → id R


(Really, L' is not very useful in this case; it would have been almost as simple to write L → id R | id R L. But in either case, you end up needing to factor out the common prefix.)