Actually grayscale won't do anything because LCD displays illuminate all the pixels even if they are black, etc (I think, I'm not an expert on this stuff). The reason a grayscale mode helps the Galaxy S5, is because it uses an Amoled display which dynamically illuminates the pixels, so black/gray pixels won't be illuminated as much, saving power.
I think that's how it goes anyway, but to put it short, I know that grayscale on LCD is pointless, grayscale on Amoled actually does save power.
ANYWAY. I have an iPhone 6 and suffer from battery life issues. I average 4-6 hours, even at 30% brightness, Spotlight is completely off except for searching apps, mail is set to manual, background refresh is off, location services are all set to never or "while using", I mostly look at social media, browse the web, or text (iMessage and SMS occasionally). Nothing too crazy. At home I am on wifi the whole time but on field test, my cell service is -108 to -118, so it's borderline on being really bad, but it works perfectly fine, nonetheless. My area does NOT have large dead zones where it says "No Service" or "1x" or anything, I at the very least have like 1 to 2 "dots" of 3G or LTE (Verizon), and it works fine. I very rarely see "low signal" under the apps listed in the battery usage list, so I have no clue what the deal is. I've tried to cope with it but it still bothers me because I have no clue why I get 4-6 hours while everyone else gets 8-10.