What is the algorithm to create colors for a heatmap?

Question

Assuming values are normalized from 0 to 1, what is the algoritm to get a color to create a heatmap like this?

1 is red, .5 is green, 0 is dark blue.

Working in

Answer 1

I leave here a Swift 4 implementation based on this blog post for any amount of colors! Perfect explanation is there! Hope it helps and saves some time to someone!

import Foundation
import UIKit

struct ColorPoint {
    let color: UIColor
    let value: CGFloat
}

class HeatMapColor {
    var colorPoints: [ColorPoint]

    init(colorPoints: [ColorPoint]) {
        self.colorPoints = colorPoints
    }

    func colorAt(value: CGFloat) -> UIColor {
        if(colorPoints.isEmpty) { return UIColor.black }

        let colorsPointsToUse = colorPoints.sorted { (colorPointA, colorPointB) -> Bool in
            return colorPointA.value <= colorPointB.value
        }

        for (index, colorPoint) in colorsPointsToUse.enumerated() where value < colorPoint.value {
            let previousColorPoint = colorsPointsToUse[max(0, index - 1)]
            let valueDiff = previousColorPoint.value - colorPoint.value
            let fraction = valueDiff == 0 ? 0 : (value - colorPoint.value) / valueDiff

            guard
                let prevComp = previousColorPoint.color.cgColor.components,
                let currComp = colorPoint.color.cgColor.components else { continue }

            let red = (prevComp[0] - currComp[0]) * fraction + currComp[0]
            let green = (prevComp[1] - currComp[1]) * fraction + currComp[1]
            let blue = (prevComp[2] - currComp[2]) * fraction + currComp[2]

            return UIColor(red: red, green: green, blue: blue, alpha: 1.0)
        }

        return colorsPointsToUse.last!.color
    }
}