Strictly speaking, that looks to be a trimetric projection, not isometric. (Both are examples of 'axonometric' projections.)

To set up an Axonometric grid, File->Document Properties->Grids, pick Axonometric from the drop down and click New. It defaults to an isometric grid, but you can adjust the angles to produce dimetric/trimetric grids (Roughly speaking, if the angles are 30°, it's Isometric, if something else, but equal, it's Dimetric, if they are unequal, it's Trimetric.)

You can actually do the colouring in Inkscape if you want, but exporting for the GIMP is just a matter of File->Export to Bitmap.