k, I'll have a shot at this
Open up the original texture file (.Bmp), or if starting a new texture, create a new image thats 512 x 512 pixels in RGB. You should have a tool bar open called 'Layers'. If not you can open it from one of the drop down menus (Windows I think its called, working from memory as Im at work). You should see one layer called 'Background'. Create a new layer by clicking on the little icon thats looks like a paper page at the bottom of that window (should be next to one that looks like a tiny bin). This will give you a new layer. To work in this layer, simply click on it a it should highlight blue. Now, everything you do on the page won't affect the original background layer, so create your stripes on this layer. Loads of ways to do this, either paint yourself, or, select the lightest colour you want in the foreground colour box and the darkest colour in the background colour box (bottom of main toolbar), run the filter Render-Clouds. Then you can muck about with the distort filters such as Glass or Wave to get something like stripe. Alternatively and much easier is if you have Eye Candy filters installed, run the smoke of Fire filters, play with the settings and colours, and instant Tiger stripes!! Thats what I did!.
If you want to affect a particular colour or change the overall colour in the image, select the layer you want to modify (if more than 1) then got to Image-modify-Hue & Saturation. This will bring up a box with 3 slide bars, Hue, Saturation and lightness (I think). You can change to overall colour using the Hue slider. The saturation will either birghten or dull down colours (to grey if need be), lightness (brightness?) does what it says on the tin. At the top of this window will be a drop down menu listing Master and all the colours. This will allow you to chose whether you want to affect all colours (master) or just the Blue (for example). If you select Blue and muck a bout with the Hue settings, you can change the blue into greens or whatever. It can take a bit of fiddling with it to get it looking right if you have gradients of colours, but you'll see whats happening and can adjust it quite easily.
Then,when your done, go to File, Save As, select Bitamp and save away. To save to original picture (including layers) save as a Photoshop (PSD) file. This way, you can come back later and edit part of the image your bnot happy with without haviong to seperate everything again. The game needs bitmaps tho
remeber, if you screw it up, just go back through the History widow to the bit before you knackered it
Phew, hope that helps rather than just confuses you!
Thu11s