Changing Colours
Not all colours used in a website template can be easily changed. Usually a web template has been designed with a particular colour scheme in mind, and the colours harmonize with the imagery used. Preferably check to see if the designer has made the template available in a range of colours, and pick the scheme that you like best. Never assume that you will be able to easily make changes in colours - you may need to use a graphics program to effect some of the changes you want. If you are unfamiliar with editing graphics, check with the template designer what changes you can make yourself, and what changes you will need to ask him/her to make.
Font colours - as long as the text is really text, and not a graphic - can be changed. This includes headings, links etc. It may be possible to change the background colour of the page, or of a table or cell - but this is not always the case. If the background colour is not solid, and has a gradient effect to it, then it is in fact created using an image. You would then need to change the background image, either by editing the existing image or substituting it with a new one.
Changing Text Colours
Text colours are easy to change, particularly if the website template uses an external stylesheet. This is a file with a ".css" extension, and it defines the various styles used in the template. Font family (i.e. Verdana, Arial, Times New Roman etc.) and size will be defined in this file, as well as text, heading and link colours. Changing the defined colour for an element in the stylesheet, changes all of those elements throughout the website.
If there is no stylesheet, then simply select the text you want to change, and pick a new colour from the appropriate tool in your HTML editor.
Changing Image Colours
Changing an image colour actually means replacing it with a new image - but you could do that by editing the existing image, and re-colouring the parts of it that you need to. Then save it with the same name. This is particularly useful if you change the background colour of your page, and you then see that some of the images do not have transparent backgrounds. You will need to change the background colour in those images too. Edit the image with your favourite graphics program, and use the paint tool to paint the background areas with the new colour.