Keyboard shortcuts must be letters, they cannot be numbers. But other than that, yeah, it's possible.
Press Alt+F11 to open the Visual Basic Editor (VBE)
Insert -> Module
Type the following code:
Then go back to Excel (just re-activate it from your taskbar), and go to View -> Macros -> View Macros
Click on the macro you just created (we named it HighlightPurple) and click Options
There you can specify a keyboard shortcut
Wash/Rinse/Repeat for other colors (be sure to change the name of the macro by adjusting the Sub ...() line)
Here are the recognized colors you can use:
vbBlack
vbRed
vbGreen
vbYellow
vbBlue
vbMagenta
vbCyan
vbWhite
xlNone
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