Basically I have noticed that alot of people ask for help on how to use photoshop.
This thread can be a kind of bible for those of you who don't know use or need more guidance with photoshop.
So first of all lets start with the basic colour change.
*****Click on the Images to make them bigger*****
Open photoshop and open the image you would like to change. I have picked an Onza T-Pro as it is pretty simple (Changing things to or from silver, black or white will be the most difficult. Also if you are going to use a photograph, try to find one without to much light on it and fairly good quality.)
Next click Layer>Duplicate Layer. This is so that there is another image that you can edit without changing the original image. If you look at the 'Layers' box you will see two boxes, one called something like Background and the other called something like Background Copy. Background Copy is the one we are going to edit and is already selected.
Next you have to carefully using the 'Polygonal Lasso' tool (to select this tool go to the lasso shape in the tool menu, click and hold and then go to the one that says polygonal lasso) select the area you want to change (You have to get close to the lines but if you don't press and hold the Ctrl key to add some more to the selected area or the Alt key to take away some).
After this there are a few different ways to go depending on the colour of the bike. The way I do it is to go to Image>Adjustments>Variations.
Here you will see some boxes with a picture of the bike, but with some colour added to them.
Say, for example, you wanted make the bike blue, you would click on the box labelled more blue a few times until you were happy with the colour. After that you can edit the colour more by going to Image>Adjustments>Brightness and Contrast to edit (you guessed it) the brightness and contrast of the picture or Image>Adjustments>Hue/Saturation to edit the colour there as well.
Also if you want to have a bike with more than one colour than go to the layers menu and, in the top box click the little eye on the end. This will hide the first colour you did but don't worry its still there. Then click on the bottom box (not on the eye). After that do the same that you did before. Then decide on the pattern you would like for instance have it in two halfs with the the middle line being vertical or horizontal or even diagonal. Or even flames if your feeling confident.
The final two stages are saving and uploading the pictures. if you have a transparent background on the pictures and you want it to show up transparent then save it as a .gif, otherwise I would reccomend .jpeg you do this by going to File>Save and selecting either .gif or .jpeg in the format drop down menu. For uploading the pictures I would recommend photobucket. All you need to do is sign up (free of charge) and you can store loads of pictures without losing them (I learnt from experience).
For other tips websites such as Digimods can be useful even though its not bikes.
Any other guides you feel need adding just post them here.