Colors are good enough with the various options for me, I watch videos more than view photos on the S5 so I value the response rate, pure blacks and contrast most. The iPhone 5s display make look good on numbers since it is only 4" so things such as brightness, uniformity etc are easy to achieve with a smaller, lower res display. Compare apples to apples (ahem), compare at least same resolution screens. I don't believe the S5 green is so bad as to make 70% appear 100%. Switch modes if you don't enjoy the saturated colors.
LCD is inherently flawed with low response rates, low contrast and blacks that can never be totally black. OLED solves those issues directly and the color accuracy is not the best since Samsung insists on colors that pop more since it wows the consumers. I'm never going to be doing any Adobe photoshopping on my S5 so if the colors can be adjusted on the fly I'm happy. You can similarly argue against LCDs for color accuracy when blacks are not true blacks and white is not pure white as well.