Sorry confused here do you want them up to 15ft max or height 18" which is it please?

Why not think deciduous, Tom and winter flowering. Lots of choice but best to use Google image to help you choose.

If you mean up to 15 feet in height for the replacement plants, Photinia 'Red Robin' and Euonymus Ovatus aureus are both evergreen with colourful leaves, although the Photinia only has red leaves when new growth is produced.
Clarification on your reference '18 inches high' would, as Drc says, be useful though.

Hi Folks,Thanks for your help 15ftx5ftx18" is a raised bed at the bottom of the garden which is 18"high hope this clears up any problem.cheers tom.

Thanks for the clarification. 18" is not very deep for tall shrubs they like to put down roots to get at water etc.
If the 18" is on soil then they could get deeper, but if its in something like a wooden bottomed frame it would not support large/tall shrubs for long?
If you have enough depth here are a few to consider:
Berberis lologensis 'Mystery Fire' 3mx3m
Berberis x stenophylla 'Golden Barberry' 4mx8m eventually
Ceanothus arboreus 'Trewithen Blue' 4mx8m in 20 years.
Cotoneaster conspicuus 'Decorus'
Cotoneaster lacteus 2-3m x 4m in 10-20 years
Escallonia 'Apple blossom' 3mx3m
Garrya elliptica 'James Roof'
Ilex aquifolium 'Alaska'
There are many more depending on the height you want have a look at Goypeadia there are hundreds to look at growing in members gardens.

Suggest also considering creating a shrub framework underplanted with a sequence of deciduous climbers to fulfill your needs.At ground level Lily of the Valley/Cyclamen/Aconites etc???