When exiting a nether portal in a cool biome such as extreme hills or taiga, the grass blocks will randomly be very green in spots, as if you are in a forest biome. This doesn't seem to happen unless you use a portal to get there, however. If you update these spots, however, they will turn to their normal color.
Note that this occurred with biome blend turned off and has not been tested with biome blend on.
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Avoma
This appears to be a duplicate of MC-173723, which hasn't yet been resolved. If you have any additional information to provide regarding this issue, feel free to comment on the parent ticket. If you feel that this issue is affecting you, feel free to vote for the duplicated ticket, to give Mojang a basic idea of how many people are impacted by the problem. Just to further inform you, this will not determine how quickly the bug gets handled or fixed.
Brevort