I want to show an indicator(image or colored text) beside a task in task tab.

You cannot have images or rich text in the Task Display Name. However you can display the priority variable as part of the title.

As an example you can have a process variable pv!priority which holds values {"Low","High"} and then customize the task display name as ("Task Title" & pv!priority[1]) for Low and ("Task Title" & pv!priority[2]) for High

Thanks, I have already done similar to what you have suggested. But I want to make it more visible by using an image or any thing other