Sorry, but I'm not sure if I understand your question.
If you have the calculated number of days until expiry, then you can use that to colour ? (if less than 70 make "red" type thing?)
I don't know about alerts but have you not already answered your own question re colouring?
You should be able to create a calculated field that's essentially a Boolean Function.
Create a calculated field named something like "EXPIRY < 70?", and make the calculation:
[# EXPIRY DAYS] < 70
Then save the field.
Drag your new field into the "COLOR" box, and it should automatically update your values.
From there, you can modify your colors by clicking on the colored box next to TRUE and FALSE, and modifying accordingly.
I saw this in a training session a couple days ago, so I'm not sure I've documented all the steps correctly. Someone else may want to verify this.
Thank you both, I managed to create a Boolean Function then assign the colours.