How can I enter Hex colour values in Excel?
This is not possible. Excel uses a version of the Color Dialog Box that only allows generation of custom colours using RGS or HLS.
However you can use an online color converter, such as Colorizer to convert from Hex values to RGB and HSL and then use the results in Excel.
What is Colorizer?
Color picker, calculator and generator with high precision and contrast test. Converts also RGB, HEX, HSL, HSV/HSB, CMYK and CIE-LAB colors and lots of other formats. In the Hex-field, you can write a known color name, too.
The Color Dialog Box
The Windows SDK provides a "standard" Color Dialog Box to choose colours. Many programs are built to use this.
Displays a modal dialog box that allows the user to choose a specific color value. The user can choose a color from either a set of basic or custom color palettes. Alternatively, the user can generate a color value by modifying the RGB or hue, saturation, luminosity (HSL) color values of the dialog box user interface. The Color dialog box returns the RGB value of the color selected by the user.
Some graphics programs (GIMP for example) that need a more sophisticated colour choosing mechanism provide their own custom dialog.