photoshop question : how can i convert certain colour to another one?

Answer 1

1- Use "pen tool" to select the area you want to change it's color
2- When you select the area go to Image>Adjustments>Hue/Saturation
3- Tick the "Colorize" box
4- Make some changes in Hue Saturation and lightness to get the color you want.

Good luck
(if you need help with the "pen too" contact me)

Answer 2

Use "replace colour" (under Image - Adjust I think). If you can't find it immediately, look in the help files (press F1) and they will tell you where to find it.

Answer 3

That avatar looks familiar ;-)