High Harmonic Generation (HHG) in a gas filled capillary provides a convenient source of coherent soft x-rays, which can be focussed using glass fibre tapers to provide an x-ray source to record diffraction and scattering images of sub-micron particles, to demonstrate the feasibility of moving down in scale to image single molecules. We demonstrate the ability to control and scan the x-ray spectrum. Control over the tuning can be achieved by using different gases, by changing the average intensity of the laser pulse or by varying the quadratic spectral phase of the laser pulse. We observe an ionization-dependent blue-shift of the fundamental wavelength that is directly imprinted on the harmonic wavelengths. The harmonic tuning is shown to depend on nonlinear spectral shifts of the fundamental laser pulse that are due to the plasma created by ionization. We can spatially resolve the x-ray spectra from a single grid diffraction image.