It should be noted that, since the failed revolutions of 1848, many German polities had adopted a uniform that was similar to the Prussian style, due to the influence and reputation of Prussian arms, known as the Waffenrock. This consisted of a single-breasted jacket with lapels, belt, straight trousers and a pickelhaube which would not look too out of place in a WWI battlefield were it not for the colour. The Prussians used a dark blue commonly known as Prussian blue. In service of other German states, the major kingdoms of Saxony and Bavaria used different main colours (dark green and light blue respectively). Sharp eyed collectors would be able spot out minor differences like the colour of cuffs and pipings or different buttons, but were you to encounter a regiment of Prussians next to a regiment of Bavarians, their uniforms would essentially look like a palette swap.

A notable exception is Wurttemberg, which adopted the Waffenrock then unadopted it in 1864 for a double breasted version that vaguely resembled the Uhlan cavalry uniform of the time. It then switched back to the common patter after 1889.

When the empire unified in 1871, only the four kingdoms of Prussia, Bavaria, Saxony and Wurttemberg had independent armies, and Bavarian, Saxon and Wurttemburger armies retained their idiosyncracies, with colour being the most noticeable division. Other equipment of more logistical import such as rifles, however, were unified. The Waffenrock was not replaced until 1907, when it was superceded by the field gray Feldrock. The M1907 pattern was updated a few times before WWI, but in general, it looked very similar to the uniform the Germans unified under, except in a uniform khaki grey. All German troops now wore the same colour for camouflage, but there were still tiny differences: notably, the Bavarians had a lion rather than a crown on their buttons and belt buckles, and the bottom of the two cockades of the field cap were different; the top one was always red white and black for the German Empire, but Saxon (green), Bavaria (blue), Prussia (black) and Wurttenburg (red) had different lower cockades.

So in summary, yes, they did eventually unify the uniforms, but it took longer than one would imagine since it was pretty close already before German Unification. Note that this only applies to the standard PFI uniform. There were enough differences between units such as artillery, infantry and notably cavalry that even within each of the constituent kingdoms there was a lot of variance.