Do you mean change the color of the car? or background? You channel may have been hidden. Go to the WINDOW menu and click on the channels selection. This should make the channel window appear.

I dont remember if photoshop 5 had this in it may have been started in 5.5 but see if there is a save for web under file menu. This will allow you to save it out as a .jpg and select the compression level and see a preview of how it will look. This is nice because you can move the compression level back and forth and see how good or bad it will look along side of the size the image.

If there is no save for web then you have to select SAVE AS under the file menu. There should be two places at the bottom for info.
1. is the file name
2. is a drop down menu that will allow you select your output format.
select .jpg format and click save.
It should then bring up another window which is your compression settings. You might have to play around with this to see what level is ok. Start off at 50%. Then open that file up in Internet explorer or back in photoshop, but make sure you are looking at it at 100% zoom or it may look pixelated.

Some tips. Always have your working .psd file the same size or larger then you ever plan on outputting it. This is because you can always shrink your image and it will keep most of its detail but you really cant enlarge the image too much before you start to notice.

Hope this helps.