Abstract: A controller for agricultural sprayers utilises a detector (23, 123, 148) to generate red, blue and green colour signals across a field of view. The colour signals are used to generate a `its green` or `not green` output to switch a spray nozzle (13) detection of something deemed to be green. The algorithm which determines if there is something which is `green`, rather than `not green`, looks at the level of green component over the red and blue components in the colour signal and if both are exceeded then the decision is that it is `green`. The level of green over each of red and blue can be compared against preset values to determine the `green`, `not green` output. The level of green can be established by summing pixel by pixel over an area within the field of view under consideration to see if the sum for the area exceeds a set level to decide that the area is `green` and requires spraying.