Is it possible to set a default field value in a Layout Rule?

Is it possible to set a default field value in a Layout Rule?

I would like to be able to set a default field value based on a layout rule, rather than a default value in the main layout screen.

Example:
if Field X is set to 'Banana' then set the default value of Field Y to 'yellow'
if Field X is set to 'Strawberry' then set the default value of Field Y to 'red'

Is this possible?

I don't want to use a workflow rule because a) we've hit our limit and b) it needs to happen straightaway whilst the user is on that screen.