Hi Vaibhav,
Once you can create the table, add the quantity to the color shelf.
This is what you have got.
Now change the type under marks pane from automatic to a square.
You need to change this in order to change the color that you applied from text to the square.
Now the field is shown as heat map and the text color by default color (white).
Now you can achieve what you want.
Hope this helps you.