Average two colors using square root, https://sighack.com/post/averaging-rgb-colors-the-right-way

averageColor.js

function averageColor(a, b) {
    let ar = parseInt(a.substr(0, 2), 16)
    ar *= ar
    let ag = parseInt(a.substr(2, 2), 16)
    ag *= ag
    let ab = parseInt(a.substr(4, 2), 16)
    ab *= ab
    let br = parseInt(b.substr(0, 2), 16)
    br *= br
    let bg = parseInt(b.substr(2, 2), 16)
    bg *= bg
    let bb = parseInt(b.substr(4, 2), 16)
    bb *= bb

    return `#${Math.round(Math.sqrt((ar + br) / 2)).toString(16).padStart(2, '0')}${Math.round(Math.sqrt((ag + bg) / 2)).toString(16).padStart(2, '0')}${Math.round(Math.sqrt((ab + bb) / 2)).toString(16).padStart(2, '0')}`
}

console.log(averageColor('f1f5f9', 'e2e8f0'))