This is a question that crops up quite frequently - people see freshly worked pale wood and want to keep it looking pale. The trouble is, in general, pale woods will darken when exposed to ultraviolet light - ie daylight - and finishes won't really stop that happening. Personally, I really like the brown colour of matured ash, but it's not the white that it shows when newly cut.
You can buy yourself a few years delay by choosing a finish which won't itself darken the colour - this could be a very pale shellac or a polyurethane. Don't assume that polyurethane has to look thick and glossy - you can use a matt or satin version and thin it down a bit for the first coat. If it's a lightly-used side table one coat followed by some wax could be enough.
There are some newer finishes designed for exterior use which contain UV blockers but I've not got personal experience of them.
As with any finishing question, it's always a good idea to experiment on off-cuts of the same wood before committing yourself.
