Unfortunately you cannot use hex color codes for custom color formatting in Coda, but it does have a custom color picker in the conditional format rule builder.
Coda Docs allows users to add color to cells and rows to highlight different data types. Here's how to add color to a cell in Coda:
Click the Options menu in the top right of the table
Select Conditional format in the side panel
Click + Add Rule
Select the column to base the conditional formatting rule on
Set the color and font options
The Coda Community has a resource that includes over 900 colors categorized by Hex Code and RGB Code with included search bar and quick sort to find main colors.