Outdoor wood is exposed to various factors, which causes greying or aging of the wood. Aging processes can be very different depending on the exposure. Parts of wood that are not exposed to external factors, e.g., they are under the eaves, are less exposed and therefore the aging process is slower. This can be accelerated by solutions based on iron ions. Thus, the wood turns grey quickly and evenly. However, the colour after iron treatment also depends on the previous aging of the wood. In our research, we monitored the change in colour depending on aging and processing. Before treatment, the samples were aged in the open air for different periods of time and then treated with a 5% solution of iron(II) sulphate. The samples were treated by immersion in the solution for 10 seconds and conditioned for 24 hours before exposure. Some of the samples were exposed to precipitation, where we determined the influence of precipitation on the colour change. We found that the aging time affects the colour change, as samples aged 5 weeks reached the closest colour to naturally aged wood. When monitoring the impact of precipitation, we found that precipitation affects the colour change, as the exposed samples changed colour more intensively and faster than the unexposed ones.