Samsung dramatically improved the color accuracy of their AMOLED displays with the S6, which is good since all their previous models were awful.
But even the S6 models lag behind the iPhone.
Greyscale Accuracy (lower is better)
iPhone 6 1.9683
Galaxy S6 Edge 3.1000
Galaxy S6 3.8800
Saturation Accuracy (lower is better)
iPhone 6 1.1929
Galaxy S6 Edge 1.3600
Galaxy S6 1.8400
GMB Accuracy (lower is better)
iPhone 6 1.7645
Galaxy S6 Edge 2.0200
Galaxy S6 2.3500
"However while the S6 edge seems to be without any issues, the S6 does have some odd display issues that can be seen in direct sunlight as seen in the photo above. To be clear about this the photo above is a simple white screen, which should make the image completely homogeneous, but instead there are two visible vertical lines and another jagged horizontal line that appear across the display. Inspection under a light microscope doesn’t really reveal what’s causing this, but the defect is quite visible in practice as seen in the photo above."
"As far as I can tell, it looks like the green subpixels have a bit more variance than what we’re used to as they tend towards oblong shapes rather than circles, which is likely due to the much tighter pixel density. It seems that this variance may cause some color shifting in certain units, which seems to remain a potential problem with Samsung's AMOLED displays."
"Due to the subpixel arrangement and some other differences in the display design, color shifting also remains higher than one would expect from LCD displays that are found in phones like the iPhone 6."
"Issues like purple smearing have been resolved, but there are still some problems with the display such as color shifting with changes to viewing angles and some variability in display quality from unit to unit."