Alright fellas im recolouring a motorcycle for someone who wants a paintjob and recolouring the coloured fairings was easy. However he wants half the tank recoloured which is black and is proving hard to recolour whilst keeping the features (shadows and whatnot). Does anyone know how to do this? Im fairly sure if i had photoshop i could do some blending with the layers however i cant be bothered installing photoshop and GIMP has done a good job up until now except it seems to be lacking in the layers department.

I have attached the original and what i've edited. Im trying to get half the gas tank (to the bottom of the "dent") to match the red top fairing. Any colour mods on the black just look ugly....

The easiest thing to do (and I'll be using Photoshop terms here since that's what I use) would be to create a duplicate layer with the original bike pic on, then hide the original. Cut out the new layer so that just the part you want to edit is left, then you can edit the layer pallette to tint what's already there rather than painting your own mask over the top.
The frontmost part of your edited paint job looks good, but the edges on the rest are a little suspect - I'm guessing you did the front bit first and it was taking too long

Once you've got your cutout part sorted, you can use the colour replacement to remove all of the blue and stick red in instead, or remove the black and make it whatever other tone you want.

Then when you un-hide the original layer, you keep the smooth edges since the two parts are from the same original photo.

I think i figured it out. Need to invert the black to white.

Bah it looks ugly see attachment. This is what i didnt want. It shouldnt be so dark especially in the center. It should be a nice red like the rest. I had the same problem of a dark patch on the upper fairing above the indicator but i sorted that out easy this one is a tough one though Its 1am so im going to bed sleep can only do me good.

I wouldn't do anything like inverting big patches of colour, it's not necessary and you'll lose all of the highlights and lowlights on the reflections. If you use the Hue/Tone/Colour editing sliders (Edit > Image Adjust > err...something in Photoshop) like I suggested before, you can keep the original highlighting and just change the colours of the parts you want.

Quote from Dajmin :

I wouldn't do anything like inverting big patches of colour, it's not necessary and you'll lose all of the highlights and lowlights on the reflections. If you use the Hue/Tone/Colour editing sliders (Edit > Image Adjust > err...something in Photoshop) like I suggested before, you can keep the original highlighting and just change the colours of the parts you want.

Except it does not work. It comes out looking very ugly. Like in the attachment above.

if i got your point, your trying to make some new colours to the bike? try brushing?first just normally do the paintjob without the shadows, and later make a new(and maybe it requires even more) layers to the shadowing. it isnt that easy to brush the shadows but its quite easy to learn. this is the way i use shadows in gimp, i find the colouring tools that come with it pretty useless. also i see you used the ''macig wand'' looking tool witch selects the area of a certain colour in the pic havent you? well try taking some time on this and not do it that way, i think you can see it also that there is this kinda white ''chain'' around the red parts. just brush them with time and care and it will be 100 times better. also if your just starting to use gimp and dont know anything, when you have a brush selected you can do straight lines by pressing shift. also dont scale the size of the brush very much, when its around 1.00 it gives the best quality on edges
edit:also if your not able to do that tank part i can do it, pm if need more help

Maybe like this?