For Plotly.js, the default color list is given in the source code:
'#1f77b4', // muted blue
'#ff7f0e', // safety orange
'#2ca02c', // cooked asparagus green
'#d62728', // brick red
'#9467bd', // muted purple
'#8c564b', // chestnut brown
'#e377c2', // raspberry yogurt pink
'#7f7f7f', // middle gray
'#bcbd22', // curry yellow-green
'#17becf' // blue-teal
For Plotly Python, see this answer.
If a plot has more than 10 series, it will cycle back to the first color.