For sleek and blunt styles:
Go for a block colour which will encourage shine and accentuate the sharpness of your cut.
A rich dark chocolate semi permanent will look great, a platinum blonde or a bright red, the more daring the better.
Remember it is best to keep a block colour instead of highlights if you want a sleek glossy look.
If you have any hair colour on your hair whether it is semi-permanent or permanent, then it can and will affect any other colour you put on your hair.
One important rule to remember is that TINT WON'T LIFT TINT, therefore if you want to go lighter and have a colour on your hair already, then you can't do this with a normal hair colour/dye. The main thing you can do is to use bleach, either in its full strength form or in a weaker form like a bleach bath
The condition of your hair plays a very important part in achieving and maintaining your perfect hair colour.
If your hair is in bad condition such as having lots of split ends, very dry and over processed with colours, then the colour may not take very well on the hair.
Also if your hair is very porous (where the cuticles are wide open, usually when there are split ends) then the colour can take really well initially but after a few washes, due to the cuticles being open, the colour washes out very easily leaving a very dull washed out look to your hair.
If you don't know whether you have a warm or a cool skin tone then there is a simple way to find out.
In daylight look at the veins in your forearms, if the veins are more green than blue, you would be classed as warm skin toned (the blue veins coming through warm skin tones will create a green tone). If these veins look blue then you would be classed as having a cool skin tone.