I really like red, purple and yellow together. It is very bold and hot. I think (from memory) it's a scheme that Christopher Lloyd is quite keen on in his book on colour in the garden.
One thing I would say is that it's good to think about year-round planting. You want this to look good in February, and super-excellent in July. I find it helpful sometimes to divide the year up like this:
Nov-December
Jan-Feb
March-April
May-mid/late June
Mid/late June-late July
August (I put this separately because I think it can be a difficult month to get right)
September-October
And then choose a few 'star performers' for each of those blocks in your colour scheme. The reason I start with November is that those are my 'structure' plants that are there to make it look green and leafy all year round - they sort of anchor the whole thing, if that makes sense, and they tend to include things that are more imposing. So for example, in one of my borders in the back garden, my November/December plants are a Pittosporum 'silver queen' (foliage) alongside a silver Corsican hellebore (flowers); in another, they are box (shape/foliage) and a Viburnum tinus (flowers), in another a fatsia (shape/foliage/flowers) and a bamboo etc etc etc.
For you, Nov/Dec could possibly be something like an Erysmium 'Bowles Mauve' (evergreen, purple flowers all winter sometimes!), purple-coloured heucheras (you can decide whether you prefer the bluer-purples or the redder ones), a golden Elaeagnus for a shot of yellow foliage, one of the more delicate new mahonias (soft caress has fern-like foliage and yellow flowers in November) and perhaps one of the variegated white/red photonias with the pinker colouring rather than the more tarnished brown red of 'Red Robin'??
Can I also recommend that you join the Secret Gardening Club and get a plant pass? They are excellent and will save you £££££ at the garden centre!!