Clan tartans usually come in two variations - 'Dress' (bright and colourful for use in the evening when dark colours wouldn't show up in subdued light) and 'Hunting' (dark sombre colours which have to be seen in daylight to show up at all.) All the military tartans were 'Hunting' as soldiers usually oporated during the daylight, but some of these were converted into 'Dress' versions (not by the military!) by adding lots of white to them (as in Dress Gordon). All the colours were originally in what we now call 'modern colours', that is a standard colour scheme where red is very bright and blues and greens are very dark. Then the tartan manafacturers developed their 'ancient colours' (light pastel shades, where the red of MacPhie would be rendered as a beautiful soft almost flame colour) and 'weatherd colours' (really dummed down with reds and blues being even lighter and softer, but greens turning brown). The House of Edgar mill now produce 'muted' shades, which have the same effect as 'ancient' but are slightly darker colours. The only drawback at present is that this colour scheme is only available in medium weight cloth. Then there's weight of cloth to consider. Nearly all of the clan tartans are available in medium weight, but only the more popular ones are available at the cheaper price in heavyweight. That's because only these popular tartans are produced in double width lengths of cloth (D/W). The others are available in this weight but are more expensive as they are only produced in single width cloth (S/W).
So, when choosing your kilt firstly decide on the weight of cloth you want. The size of the sett can alter the look of a tartan, so don't forget that sett size is slightly smaller in medium weight. Do you want a real 'tank' in heavyweight or a lighter medium weight ? Then choose your tartan and consider if it would look best in 'modern', 'ancient' or 'muted/weathered' shades (this is a big factor to consider if you want to wear your clan tartan and no other). The main factor that makes a 'real' kilt is the pleating. In order for the kiltmaker to really get the pleats looking good he/she must have at least 8 yards of cloth to work with. The cheap polyviscose kilts look good at the front, but the pictures on ebay don't show the rear of the kilt ! A full set of pleats not only looks good but they 'swing' as well, and this is one of the real joys of wearing a kilt. You really have to show off in your kilt, sport the tartan and swing the pleats !
The other X markers have all given good advice so the rest is up to you. Good luck !