The 19th century ended with every army dressed in a drab colour. It had started with red coats and blue coats and white coats. Here is the hundred year journey.
Quick answer. 19th century infantry uniforms moved from the brightly coloured coats of 1815 (British scarlet, French blue, Russian green, Prussian dark blue) to the drab khaki, grey and horizon blue of 1914. The transition was driven by rifled firearms making coloured uniforms suicidal in battle. Along the way the long tailed coat became the short tunic, the shako became the kepi or helmet, and the full dress uniform became reserved for parade only.
British infantry
British infantry wore scarlet throughout the 19th century, with slow modifications. The 1812 Belgic shako gave way to the 1829 bell topped shako, the 1844 Albert shako, the 1869 Austrian pattern shako and the 1878 home service helmet. The coat moved from the coatee of 1800 through the 1829 double breasted tunic, the 1855 single breasted tunic, and finally the full dress tunic of 1902. By the Boer War khaki had replaced scarlet in the field; by 1902 the transition was universal. Scarlet survived for ceremonial.
French infantry
French infantry wore dark blue tunics with regimental facings through the 19th century. The red trouser, adopted during the Second Empire, became iconic and was retained unto the Franco Prussian War (1870 to 1871) and beyond. French infantry entered the First World War in dark blue and red, with grey replacing them during the war itself (horizon blue in 1915, grey blue later). The Zouaves, Chasseurs d'Afrique and other colonial units added colour. The French dress of 1914 is often shown in First World War photographs with the men scrabbling for life in trenches wearing clothes designed for parade.
Prussian and German infantry
Prussian infantry wore the dark blue tunic (Litewka) with regimental facings and the pickelhaube spiked helmet from 1842. The dress survived through the German unification wars and into the First World War. By 1910 field grey replaced dark blue for everyday field service, though full dress remained dark blue. The pickelhaube continued until replaced by the stahlhelm steel helmet in 1916. German infantry of 1914 wore field grey with the pickelhaube, which from a distance read as a drab silhouette with a spike on top.
American infantry
The United States Army wore dark blue through the century. The Mexican War (1846 to 1848) was fought in dark blue with white or sky blue trim. The Civil War (1861 to 1865) continued the pattern for Union forces, with the Confederate Army in grey or butternut. The post Civil War army wore dark blue for garrison and khaki for colonial campaigns. The Spanish American War (1898) saw the full transition to khaki as the standard American field uniform. By 1914 the US Army had the dark blue dress uniform and the khaki field uniform in parallel.
Russian infantry
Russian infantry wore dark green with red facings through most of the 19th century. The Crimean War did not change the pattern much. The Russo Turkish War (1877 to 1878) saw white winter wool uniforms and dark green summer. The 1881 pattern simplified the coat and introduced the furashka cap. By 1908 khaki (actually a drab brown green) was introduced for field service, and the Russian army entered the First World War in khaki with the furashka cap.
The 1914 picture
The infantry of August 1914 presented a transitional picture. British, Russian and German infantry wore drab colours (khaki, drab, field grey). French infantry still wore dark blue and red. Austrian Hungarian infantry wore pike grey. Italian wore grey green. Only the French, among major European armies, had not completed the transition to field dress. The result was heavy French casualties in the opening weeks of the war, and the rapid introduction of horizon blue by 1915.
Common questions
Why did infantry uniforms get less colourful?
Rifled firearms made coloured uniforms suicidal. By the 1880s British soldiers in scarlet were being shot by Boer riflemen at 1,000 yards. Khaki, grey and other drab colours offered concealment at range.
When did Britain adopt khaki?
For colonial service from the 1880s. For home service in 1902 after the Boer War.
When did France give up red trousers?
1915. French casualties in the opening weeks of the First World War forced the transition to horizon blue.
What was field grey?
The grey green colour adopted by the German army for field service from around 1910. Officially feldgrau.
Where can I see 19th century infantry uniforms?
Major military museums in London, Paris, Berlin, Moscow and Washington all hold extensive 19th century infantry collections.