f1capsicum wrote:
Can anyone recommend an easy Allium for me?
Preferably colourful (red or purple) and 2-3 ft in height.

They're all easy. In your height range, Allium aflatunense is pink,
albopilosum is sort of pale lavenderish; giganteum gets to four feet
and is sort of mauve. The small ones are very nice, too.

When should Allium's be planted and how do they stand up to slugs
and
other pests?

Autumn; they seem pretty tough, pest-wise -- beware of the same
things as for onions. But they don't like being overshadowed, and
will die out under those conditions -- or they have for me, anyhow..

Do they deter cats at all?

Not in my experience.

--
Mike.