You can pack a real punch even in a small garden by getting maximum impact from certain features. Pick one plant in your favourite colour and plant a block of it, closely packed, and you'll really grab people's attention.Garden Design - Plant blocks of colour for impact in a small garden
A single decent sized specimen tree is usually better than several small ones when designing for a limited space. A restricted variety of plants often appears more coherent than trying to cram too many in. And when it comes to colour it can be a good strategy to be bold with a few chosen colours rather than attempt too many combinations close together.
Here are some examples:
Daffodils in Thompson's Park, Cardiff in March stretch impressively and naturally into the distance. 
A designed version of the same effect with a long narrow bed of roses in Mirabell Gardens in Salzburg, a block of colour created by a procession of blooms along the line of vision. Tulips also work brilliantly in this way.
GREEN FINGERS TIP: You can achieve this effect even in a smaller space and with fewer flowers, as long as they are in a line running away from the viewer. You can even have groups some distance apart, which will still appear as a block of colour if they are in line.
Blocks of Lavender, one behind the other, at St Fagans in Cardiff. Combined with the box hedging makes a clean and simple effect using just two colours.