It's more or less impossible to alter the colour of cast concrete without the use of expensive chemicals, and even then, it's a bit hit and miss.
Your flags are best left alone. Sealants act as so-called "Colour Enhancers" and will only deepen the grey tones, but, if you wait for long enough, UV light will, over a period of some years, bleach the flags and lighten their colouring.
I really can't say. I haven't seen the flags, or their location, so whatever I say would be a guess. However, it's unusual for flags to get darker once they're 12 months old: they more commonly go lighter after that period of time.
