Sorry that you cannot color code keywords in Photoshop Elemensts.
Which version of Photoshop Elements are you using and what is your computer's OS? Here's the general workflow to edit/add a color tag to Keword.
Click the New button and select Edit from the menu.
Click Edit Icon in the Edit Keyword Tag dialog box.
You can edit the following properties of an icon:
Color: Click Choose Color and select a color to change the color of the icon.
Let us know if the above workflow does not work for you.
If the Color picker is still grayed out, restore the app preferences to default as mentioned below:
Go to Edit > Preferences (Mac: Photoshop Elements > Preferences > General), click the Reset Preferences on next launch button, and then click OK. When you restart Adobe Photoshop Elements, all preferences are reset to default settings.