Grass sides look strange on 'fast' graphics.

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This is intended when on fast graphics.

The colour is the same (green), the hue is different.

The effect is better visible in biome desert, see screenshots.

This is an intended behavior. The top of the grass blocks are colored according to the biome. The sides are colored with an overlay but this overlay is removed for performance reasons in "fast" mode. You are right, this looks strange. But I don't think there is an easy way to fix this.

But you get what you pay for. If you want fancy graphics, you have to select "Fancy graphics".
This is just one of the tradebacks for "Fast graphics"

What's confusing about this is that apparently the top is biome-colored in fast, but the sides are not. Why is the top still colored in fast, if it's an expensive operation? If the method used for the top is different, and isn't an expensive operation, then why isn't that same method used for the sides?

It would probably look better if it were at least consistent.

Fixed in 14w25a