Warning up front, if you are trying to use more that 20 individual colors people will not be able to tell the difference. In addition Tableau repeats color use when you get over that number of categories.
In your sample you attached:
1) Drag 'Rank Sales' to the color box in the Marks section.
2) select the down arrow and change 'automatic' to 'square'
3) select size button and drag the slider to the right. this changes the size of the color to include the entire cell.
4) the default color is going to be single color range from 1 to 1263, if you select the color button, edit button, you can change the colors of the cells. Try Red-Green-Gold diverging, also check the box for reversed.
Step1: Drag Rank(sales) to text and color
Step2: on marks card select sqaure instead of text.
Step3: Right click on Rank(sales) which is on color card and select discrete.