Style lines using an identity property function.
Using an identity line-color property function for a visualization
<!DOCTYPE html>
<html>
<head>
    <meta charset='utf-8' />
    <title></title>
    <meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
    <script src='https://api.tiles.mapbox.com/mapbox-gl-js/v0.39.1/mapbox-gl.js'></script>
    <link href='https://api.tiles.mapbox.com/mapbox-gl-js/v0.39.1/mapbox-gl.css' rel='stylesheet' />
    <style>
        body { margin:0; padding:0; }
        #map { position:absolute; top:0; bottom:0; width:100%; }
    </style>
</head>
<body>

<div id='map'></div>
<script>
mapboxgl.accessToken = '<your access token here>';
var map = new mapboxgl.Map({
    container: 'map',
    style: 'mapbox://styles/mapbox/streets-v9',
    center: [-122.48383155304096, 37.82882682974591],
    zoom: 16
});

var red = '#F7455D';
var blue = '#33C9EB';

map.on('load', function() {
    map.addLayer({
        'id': 'lines',
        'type': 'line',
        'source': {
            'type': 'geojson',
            'data': {
                'type': 'FeatureCollection',
                'features': [{
                    'type': 'Feature',
                    'properties': {
                        'color': red,
                    },
                    'geometry': {
                        'type': 'LineString',
                        'coordinates': [
                            [-122.4833858013153, 37.829607404976734],
                            [-122.4830961227417, 37.82932776098012],
                            [-122.4830746650696, 37.82932776098012],
                            [-122.48218417167662, 37.82889558180985],
                            [-122.48218417167662, 37.82890193740421],
                            [-122.48221099376678, 37.82868372835086],
                            [-122.4822163581848, 37.82868372835086],
                            [-122.48205006122589, 37.82801003030873]
                        ]
                    }
                }, {
                    'type': 'Feature',
                    'properties': {
                        'color': blue
                    },
                    'geometry': {
                        'type': 'LineString',
                        'coordinates': [
                            [-122.48393028974533, 37.829471820141016],
                            [-122.48395174741744, 37.82940826466351],
                            [-122.48395174741744, 37.829412501697064],
                            [-122.48423874378203, 37.829357420242125],
                            [-122.48422533273697, 37.829361657278575],
                            [-122.48459815979002, 37.8293425906126],
                            [-122.48458743095398, 37.8293447091313],
                            [-122.4847564101219, 37.82932776098012],
                            [-122.48474299907684, 37.829331998018276],
                            [-122.4849334359169, 37.829298101706186],
                            [-122.48492807149889, 37.82930022022615],
                            [-122.48509705066681, 37.82920488676767],
                            [-122.48509168624878, 37.82920912381288],
                            [-122.48520433902739, 37.82905870855876],
                            [-122.48519897460936, 37.82905870855876],
                            [-122.4854403734207, 37.828594749716714],
                            [-122.48543500900269, 37.82860534241688],
                            [-122.48571664094925, 37.82808206121068],
                            [-122.48570591211319, 37.82809689109353],
                            [-122.4858346581459, 37.82797189627337],
                            [-122.48582661151886, 37.82797825194729],
                            [-122.4859634041786, 37.82788503534145],
                            [-122.48595803976059, 37.82788927246246],
                            [-122.48605459928514, 37.82786596829394]
                        ]
                    }
                }]
            }
        },
        // The identity function does not take a 'stops' property.
        // Instead, the property value (in this case, 'color') on the source will be the direct output.
        'paint': {
            'line-width': 3,
            'line-color': {
                'type': 'identity',
                'property': 'color'
            }
        }
    });
});
</script>

</body>
</html>
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