In germany, the privatisation and segmentation of the railway company has created many "interface points" where private entities can rent vehicles or use the tracks. This resulted in a vast amount of companies specializing in engine renting or vehicle transport, like Captrain/ITL or MRCE. The diversity in companies results in a big variety of color schemes.
Train museums use these possibilities to run extraordinary trains [1]. Rarely you can see them as replacement trains in regular service[2].