vis.js is great. Here are the colours it uses for traces by default:

blue: #4f81bd
orange: #f79646
green: #75c841
purple: #8c51cf

Update: found a much longer list in the source code:

var htmlColors = { black: '#000000', navy: '#000080', darkblue: '#00008B', mediumblue: '#0000CD', blue: '#0000FF', darkgreen: '#006400', green: '#008000', teal: '#008080', darkcyan: '#008B8B', deepskyblue: '#00BFFF', darkturquoise: '#00CED1', mediumspringgreen: '#00FA9A', lime: '#00FF00', springgreen: '#00FF7F', aqua: '#00FFFF', cyan: '#00FFFF', midnightblue: '#191970', dodgerblue: '#1E90FF', lightseagreen: '#20B2AA', forestgreen: '#228B22', seagreen: '#2E8B57', darkslategray: '#2F4F4F', limegreen: '#32CD32', mediumseagreen: '#3CB371', turquoise: '#40E0D0', royalblue: '#4169E1', steelblue: '#4682B4', darkslateblue: '#483D8B', mediumturquoise: '#48D1CC', indigo: '#4B0082', darkolivegreen: '#556B2F', cadetblue: '#5F9EA0', cornflowerblue: '#6495ED', mediumaquamarine: '#66CDAA', dimgray: '#696969', slateblue: '#6A5ACD', olivedrab: '#6B8E23', slategray: '#708090', lightslategray: '#778899', mediumslateblue: '#7B68EE', lawngreen: '#7CFC00', chartreuse: '#7FFF00', aquamarine: '#7FFFD4', maroon: '#800000', purple: '#800080', olive: '#808000', gray: '#808080', skyblue: '#87CEEB', lightskyblue: '#87CEFA', blueviolet: '#8A2BE2', darkred: '#8B0000', darkmagenta: '#8B008B', saddlebrown: '#8B4513', darkseagreen: '#8FBC8F', lightgreen: '#90EE90', mediumpurple: '#9370D8', darkviolet: '#9400D3', palegreen: '#98FB98', darkorchid: '#9932CC', yellowgreen: '#9ACD32', sienna: '#A0522D', brown: '#A52A2A', darkgray: '#A9A9A9', lightblue: '#ADD8E6', greenyellow: '#ADFF2F', paleturquoise: '#AFEEEE', lightsteelblue: '#B0C4DE', powderblue: '#B0E0E6', firebrick: '#B22222', darkgoldenrod: '#B8860B', mediumorchid: '#BA55D3', rosybrown: '#BC8F8F', darkkhaki: '#BDB76B', silver: '#C0C0C0', mediumvioletred: '#C71585', indianred: '#CD5C5C', peru: '#CD853F', chocolate: '#D2691E', tan: '#D2B48C', lightgrey: '#D3D3D3', palevioletred: '#D87093', thistle: '#D8BFD8', orchid: '#DA70D6', goldenrod: '#DAA520', crimson: '#DC143C', gainsboro: '#DCDCDC', plum: '#DDA0DD', burlywood: '#DEB887', lightcyan: '#E0FFFF', lavender: '#E6E6FA', darksalmon: '#E9967A', violet: '#EE82EE', palegoldenrod: '#EEE8AA', lightcoral: '#F08080', khaki: '#F0E68C', aliceblue: '#F0F8FF', honeydew: '#F0FFF0', azure: '#F0FFFF', sandybrown: '#F4A460', wheat: '#F5DEB3', beige: '#F5F5DC', whitesmoke: '#F5F5F5', mintcream: '#F5FFFA', ghostwhite: '#F8F8FF', salmon: '#FA8072', antiquewhite: '#FAEBD7', linen: '#FAF0E6', lightgoldenrodyellow: '#FAFAD2', oldlace: '#FDF5E6', red: '#FF0000', fuchsia: '#FF00FF', magenta: '#FF00FF', deeppink: '#FF1493', orangered: '#FF4500', tomato: '#FF6347', hotpink: '#FF69B4', coral: '#FF7F50', darkorange: '#FF8C00', lightsalmon: '#FFA07A', orange: '#FFA500', lightpink: '#FFB6C1', pink: '#FFC0CB', gold: '#FFD700', peachpuff: '#FFDAB9', navajowhite: '#FFDEAD', moccasin: '#FFE4B5', bisque: '#FFE4C4', mistyrose: '#FFE4E1', blanchedalmond: '#FFEBCD', papayawhip: '#FFEFD5', lavenderblush: '#FFF0F5', seashell: '#FFF5EE', cornsilk: '#FFF8DC', lemonchiffon: '#FFFACD', floralwhite: '#FFFAF0', snow: '#FFFAFA', yellow: '#FFFF00', lightyellow: '#FFFFE0', ivory: '#FFFFF0', white: '#FFFFFF' };