Which lure colors should I use?
This is dictated by many things: Species, Location, the day, the fish, Superstition, and what makes you feel confident.
Many people will tell you to use bright colors on Sunny days and darker colors on overcast days. While this may hold some truth, I will usually start the day with four or five different colors and then double up on the color that gets its second hit first increasing my chance for a double header.
In 1980 I was the mate on the winning boat at the Atlantic City Marlin Tournament giving my anglers seven shots at White Marlin each of the two days. The first day, I settled in using two whole squid (pinkinsh in color) and two Bonita strips on the flat lines with 1/4oz pink Sea Witches. We saw lots of Tuna and Dolphin as well as the Whites that day as well.
The second day, neither Pink or Squid raised anything and I settled in with Swimming mullet on the flat lines and Ballyhoo with Purple Sea Witches on the rigger baits. Most other boats at the time were dragging "Green Machines" as the other mates could not rig baits and the Green Machine was one of the only lures at the time considered to be good for catching Marlin (in New Jersey). It is no wonder that we kicked butt as we were using the proper colors rather than just green. Fishing out of Ocean City, MD the same year, we didn't fare as well as these competing boats had much more experienced crew all doing what I was, but they found the congregated fish while we did not.
In any case, my last Dolphin trip ended up with four Dolphin and a Barracuda eating our Blue & White Squid/Cone Head Daisy Chain, one Dolphin on a Blue & White Steel Head, One Dolphin on a 9" Green Tuna Bandit, and one Dolphin on a Rainbow Squid/Tuna Bandit Daisy Chain. This will certainly show that the fish preferred Blue & White that day. All of these fish were caught in less than 1.5 hours while on our way to the Bahamas at 25 miles off the coast of Palm Beach. If we had kept fishing, I would have placed at least one more blue & white lure out, and this probably would have led to a double header or two.
Another trip, I had seven out of eight fish caught eat the same Mackerel colored Bird/Bone Head Chain while the only other trolling fish caught was on a Rainbow 9" Spreader Bar rig. These patterns have proven themselves time and time again for me and if you start fishing with this in mind, I am sure you will be able to discover (and take advantages) of the daily patterns.
My suggestion is to always have enough of each color lure to have it out on every line with spares. The basic lure color patterns that I have on board at all times include:
Pink & White