IS this a simple sentence or compound sentence?

Your example Love is like a bullet: It kills everyone on its way has two simple sentences and a colon that is separating the two independent clauses. The second clause expands on the first.

A compound sentence is basically two or more simple sentences joined together by coordinating conjunctions, semicolons, or colons.

"One complete idea" is not a very good description of an independent clause.  It's like "person, place or thing" as the description of a noun.  These are first approximations, not rigorous definitions.

Another approximation for an independent clause is "something that can stand as a complete sentence".  That is somewhat circular, since we define complete sentences, simple sentences and compound sentences in terms of the clauses and relationships between them that those sentences contain.

We can break out of the circle in this way:  Subordination is whatever lets one clause act as the constituent of another.  An independent clause is a finite clause without subordination.

Having established all that, we still lack a definitive answer to the question.

 

Love is like a bullet:  It kills everyone on its way.

As written, these are two complete sentences -- two simple sentences, because each sentence contains only one clause.

I have very little time to learn the language: my new job starts in five weeks.

Here, this is written as one sentence.  The only significant difference is the lack of a capital letter after the colon.  If it's one sentence, then it's a sentence with two coordinate independent clauses -- a compound sentence.

All three of their children are involved in the arts:  Richard is a sculptor, Diane is a pianist, and Julie is a theater director.

Here, we can only guess.  "Richard" is a proper name.  The capital letter here tells us nothing about whether a new sentence has started.  This could be one single compound sentence, or it could be a simple sentence followed by a compound sentence.  Either way, there are four independent clauses.

 

A college degree is still worth something: a recent survey revealed that college graduates earned roughly 60% more than those with only a high school diploma.

This is a more complicated example.  It is complex.

A simple sentence contains one clause.  A compound sentence contains more than one independent clause.  A complex sentence contains at least one subordinate clause.

As written (with the lower-case "a") this is one sentence.  The colon coordinates an independent clause with a matrix clause, so the sentence is compound.  Additionally, there is a matrix: the clause after the colon contains another clause, a subordinate clause.

On its own, "A recent survey revealed that college graduates earned roughly 60% more than those with only a high school diploma" contains two finite clauses, one with the finite verb "revealed", the other with "earned".  If it were written as a sentence on its own, it would be a complex sentence.

The complete original sentence here is both compound and complex.  It has coordinate clauses and a subordinate clause.