You need to make sure all your detail, shading and weathering effect layers are above the yellow stripes in the layer order. Its no good doing that stuff and then putting some new stripes above it.
You could try desaturating the yellow to make it less bright. In PSP you would select the yellow area, go to the Adjust menu, then Hue and Saturation, then Hue/Saturation/Lightness. In the resultant window experiment with negative values for the saturation slider. A setting of -100% will turn the yellow totally grey so I'd guess you would need something closer to -10 to -20%.