Yellow paint shades are worse than reds for fading.
Where as reds turn to pink if not looked after, yellows can almost fade to white and in the case of metallic yellow/citrus reduced to almost silver through deterioration and UV. Pained platics (bumpers usually) tend to suffer mainly. Im assuming that the paint shop has just painted the panel(s) a left a stark edge as opposed to blending the fresh paintwork into the old panels?
So you can do some correction whilst your efforts will certainly help, it will not totally eradicate the issue and some elbow grease and hard work will be required.
As outlined previously (and this applies to your old paint, not the new), wash the car and clay it. Clay Bar can be bought from high street outlets and you can use a mild soapy water for the lubricant if you want. Autoglym have however, just introduced a clay lube and Meguiars have done them it in a kit for a while I believe so finding one shouldnt be difficult:
http://www.autoglym.com/enGB/product-proddetail.asp?v06VQ=EIEL
After claying, rinse then dry. The next step is a polish or pre-was cleanser, I personally prefer Dodo Juice Lime Prime due to the high oil content and this will give your older paintwork extra gloss;
http://www.dodojuice.com/lime-prime-pre-wax-cleanser.html
Once applied, follow instructions for hazing the buff to a finish.
The next stage is the wax. There are 100s if not 1000s to chose from, but remember that you get what you pay for. A paste or hard wax will be more beneficial and the higher the carnauba content, the better the results. Mt personal preference is Dodo Juice Supernatural but its pricey, Colinite 476s is reasonably priced, easy on/off and durable but it depends what you want to pay. Avoid the liquid waxes as they are inferior quality and wont achieve the effect that you require.
Have a look on line for car detailing products such as cleanyourcar.uk and detailing world, etc.
