You can change the colors for each range in the legend(click on an item and it opens a color picker), so you can select red for higher and blue for lower, but yuou won't get gradients unless you define ranges for each gradient. It's a bit weird with the data ranges, you're right, if you want gradients.
I'd recommend adding an enhancement request here: https://github.com/elastic/kibana/issues as this seems like a very useful color schema.