To place flowers tastefully, stick to two or three colors plus white/green (natural stems/filler colors). My favorite color combinations are pink, white and green / lavender, blue and white.
Next, place the largest size flower (for example, roses) in the vase. Add the rest of the stems by size- from largest to smallest- and finish by placing filler green or white stems anywhere you see gaps.
Alternatively, a single stem in a tall, thin vase can look elegant, especially when clustered with other single stems in vases (see photo below).
Here are some photographs for inspiration
Enjoy!
xoxo