"the Firstborn" (Colossians 1:15,18)

With regard to Colossians 1:15 and 1:18, even the though the Greek word is the same in both places (protokos...I assume its where we get our word prototype from), I think it's referring to different aspects of being the Firstborn.
Verse 18 seems very clear that it's referring to His resurrection since it says "the firstborn of the dead" and then goes on to say "that in all things He might have the preeminence." This agrees with Romans 1:4 which says that Christ is "declared to be the Son of God with power by His resurrection from the dead." Psalm 2:7 (and quoted in Hebrews 1:5) says "Thou Art my Son, this day have I begotton Thee", and since the Father eternally exists with the Son and the Holy Spirit, this cannot be referring to a precedence of the Father to the Son, but of the resurrection of Jesus from the dead. Also, in Psalm 89:27 it says "Also I will make Him my firstborn, higher than the kings of the earth." Again, we see the theme of preeminence due to His resurrection from the dead . Also, though the resurrection works in less explicitly, in Philipians 2 it says that because of His obedience to the Father (and subsequent death, burial, and resurrection) that God "hath highly exalted Him, and given Him a Name which is above every Name, that at the name of Jesus every knee should bow and every tongue should confess that Jesus Christ is Lord to the glory of God the Father..." Also, Revelation 1:5 says that Jesus Christ is the "firstbegotton of the dead" (also translated protokos). All that to say that 1:18 is very clear to me, that it's referring to the resurrection of Christ.

As for 1:15, I feel like the context points to the emphasis being on the eternal nature of the Godhead rather than specifically the resurrection from the dead. Psalm 90:2 says "from everlasting to everlasting Thou Art God" and Psalm 102:24-27 says that "Thy years throughout all generations...Thou shalt endure...Thou Art the same...Thy years shall have no end." Jeremiah wrote that "...He is the former of all things." (10:16)
In both Genesis 1:1 and John 1:1, the text opens with the eternal existence of the Godhead.
 Christ preceded the existence of all things, as Hebrews 7 says, "having neither beginning of days nor end of life."
If you take verses 15-17 as a complete thought, it's as if Paul lays out a thesis statement in 15, goes into more detail in verse 16, then summarizes in 17 by saying that He is "before all things" and that all things were brought into and sustained in existence because of Him.

In summary, I think verse 15 is referring to the eternal nature of God, and verse 18 refers to His resurrection.