Since you are very new to Tableau, I would request you to go through the shelves in Tableau. As you can see that you have put College ID in the color section. Since this is the Dimension (textual column), it will assign each and every id separate color. If you put any of measure (Numeric values like Sales, Profit, Discount, Number of Records etc) in Color, it will create a color scale (its color can be customized as per your need) and it will be applied to each and every bar depending on the value.
Eg. (Light Blue to Dark Blue)
I hope this explanation will help you in understanding, how it works. What shin has replied will surely help you out with your requirement.
You can read more about heatmap on below link. Above is the simple bar chart with the effect of heat-map depending on density of the measure.