there are loads of clematis to chose from so do some browsing and select the ones you like the best. as for colour combos it really is down to personal preference.
there are several specialsist nurseries to browse on the net.

Yes thanks I have browsed endlessly I just thought someone may have had a fantastic colour combination they could suggest that I could check out . Cheers

Karensusan wrote a fantastic blog last year on Clematis, with some stunning pictures and great advice, take a peek, I am sure you will get some inspiration from it.

The problem with planting two clematis together that flower at different times is the pruning regime. If they both require pruning at the same time, no problem. Otherwise, you've got to sort which stems to prune and which time

I'd go for a couple of the Raymond Evison varieties, quite long flowering, with an easy pruning regime - prune hard in spring. Look at C. 'Cezanne', 'Diana's Delight', 'Ice Blue', 'Rebecca', and the quite unusual 'Cassis' which I used in combination with 'Cezanne'.

'Arabella' is another good one - relatively, compact and flowers for months

If you want them to flower at the same time, get an early purple or deep red one, depends on how much contrast that you prefer (Pink will be swallowed by montana). As mentioned by Andrew, this is easier option as you can prune them at the same time (after flowering), the other benefit is the early clematis will give you second flush of flower in late summer although not as showy.
If you want them to flower at different time, the colour will depend on the other plant in your border. You will need the late clematis type, I would suggest Rouge Cardinal for long flowering period or The President for stunning large purple flower. You can also try the tulip flowering type like Princess Diana or the new Zoprika (both will flower very late summer or early autumn).
You will need to prune them hard early spring (before flowering), be careful not to cut the montana though or you will lose the flower that year

Thank you all so much. As weather is so awful I can relax and browse through my books and check out all your suggestions without feeling guilt!. Thanks again.