A FEW SUGGESTIONS:
Consider leaving your car at Oban and renting a bicycle from Archie MacConnell (01951 200355).
Try to make sure that you stay long enough to see at least one ferry leave without you.
Be up at dawn at least once - not least, for the Dawn Chorus.
Remember to apply powerful sun-block even on overcast days.
If you wish to visit Oransay, make sure to be here at or soon after Full Moon or New Moon (so as to have the best tidal window).
THE FERRY SERVICE
To reach Colonsay, you will almost certainly choose to travel by CalMac Ferries - and almost certainly you will come via Oban, which is easily reached by road, rail or bus.
Ferries from Oban to Colonsay run on Monday, Wednesday, Friday and some Saturdays in winter and there are daily sailings in summer. The crossing takes about 2 hours and 20 minutes, the ships are large and comfortable car-ferries and have full catering facilities on board.
There is another possible ferry route, via Islay, but it is really of interest mainly to people choosing to visit a number of islands. This route originates at Kennacraig, a few miles south of Tarbert (Kintyre Peninsula), which has no accommodation, no train service and very inconvenient bus services. The only real reason to use this route would be so as to include Islay and Jura in your trip, or perhaps as part of a scenic journey from Glasgow via Arran. For normal purposes, Oban is the main port.
Timetables are increasingly complex but comprehensive information can be obtained from http://www.calmac.co.uk/timetables/
You will find that there is an option to print off a pdf version of the Colonsay timetable
There is also an excellent air-service, which operates from Connel Airport (a few miles north of Oban) and which also provides a link with Islay. The aircraft is a twin-engined "Islander" and is very safe and reliable, governed by CAA regulations. The flight is short but absolutely exhilarating giving one excellent views of the scenery and, of course, a brilliant opportunity to admire Colonsay and its incredible beaches. For practical reasons you are unlikely to arrive by air as a vacationer but you really should consider taking a flight as part of your holiday, the roundtrip from Colonsay is about halfprice - it really is a great experience!
Bookings and full information are available from Hebridean Air Services