Hello Jaswanth Moolam,
Those 8 colors are the ones supported in the application. I do not suggest modifying these behind the scenes. I'd be interested in why you would need or want more than 8 colors for tabs though? In addition, you could create an Idea in the community to suggest this feature to the Tableau developement team. Be sure to add your own vote to the idea if you choose to go this route. Many idea creators forget to vote for their own idea.
Be sure to include why and how this increased capacity of colors feature would be used and why other Tableau users might find it useful.
This would not make sense unless working with a lot of dashboards. I currently create around 12-15 dashboards in my workbook. I color my dashboard along with each of its worksheet.
If I need to add any filter to a selected worksheet, it would be easy to just check mark the worksheets knowing which dashboard it's related to based on the color. I don't need to look twice on the worksheet and see if it's present in a particular dashboard. Say, 8 worksheets in each of 12 dashboards would make things frustrating. This would just save the time.
I think its already in the ideas section but I will have a look at the section again. Hope it makes sense.