Using Google Sheets for conditional formatting based on a range's own values is simple. Formatting based on another range of cells' values is a bit more involved. Following are step-by-step instructions to format a range of cells using values in another range of cells.
What is conditional formatting
Conditional formatting is used to highlight to the reader of the spreadsheet certain values that meet certain criteria. It can be used to show items over/under a certain amount, later/earlier than a certain date, or only the widgets sold by Joan.
Conditional formatting based on another cell
Our objective in this example is to highlight all of the dates on which Joan had a sale.
Select the range that you want to have highlighted. Even though you are going to be referencing another range of cells, you still only need to select the range that you are going to be formatting. In this example, we'll be highlighting the date column.
Click Format in the top menu then Conditional formatting...
The Conditional Formatting menu option will pop up a Conditional format rules menu on the right side of the screen (on the desktop version of Sheets).
Click the plus sign to begin adding the rule. In the drop-down menu for Format cells if choose the last option which is Custom formula is.
In the box that appears below Custom formula is enter
=B1="Joan". Note that you have to start the formula with a = sign just like any other spreadsheet formula. Also, note that you need to surround the word Joan with "s since it is a word instead of a number.
Now you have the sales data with dates highlighted on which Joan made sales.
Extending to the entire row
If you want to highlight the entire row based on the value of one cell, continue reading here.