Purchasing uplighting for ownership is quite expensive, so we would suggest renting. Most wedding lights rental stores will charge ~$150 for 4 uplights for 1 - 2 days of renting. This does not include higher end models which you can program (to work with the acoustics of the venue or an electronic trigger) and with many different colors. At ProLightingRental, we carry multiple Chauvet uplight models, which can be rented for as low as $17/ day.
At a macro level, uplighting refers to lights that are shining upwards usually perpendicular to a wall. Downlighting refers to lights shining downward, so they need to be installed on a ceiling or rigging to be effective. From a party planning stand-point, uplighting is much more affordable, efficient and easier to set-up and tear down than downlighting. Most wedding venues don't have the set-up for portable downlights.
As a rule of thumb, assuming everyone is seated comfortably in tables of 8, within a rectangular or square venue, for your first 50 guests - you'll want 15 - 18 lights. As you add more people, the amount of lights does not need to increase proportionally. For every 50 more guests, add between 5 - 10 more lights.
Draping and Uplighting compliment each other very well, especially when you have unsightly walls or no walls at all at your venue. Plan out the number of uplights and drapes you want first. Afterwards, set your uplights to compliment your drapes (or vice versa) and you can change a school gym into an extravagant wedding hall.
Uplights are more focused and better with "column-ing" at your venue. For example, if you want your focal point to be the bride and groom, consider putting an uplight behind them. Washlights have a much wider field of light range, but can have lenses added to them to focus similarly to uplights. Washlights are rarely built to change color, whereas modern uplight models regularly have color rotation and preset options and programming.
This all depends on your venue. Natural lighting, space, color of the walls, tables and layout of the reception are all important. However, contrary to pictures - red, blue, purple and other darker more royal hues should be reserved for dancing, after-party and more "movement" phase of the reception. Try using lighter or brighter tones like amber, light gold or biege to help lighten up your special day, during the dinner and cocktail hour. More modern uplights like our Chauvet models, come with different channels to allow for different colors and color mixing, without the need for lens.