If you have selected the optimised map as the biomass layer to monitor crop development, the colour scale ranges from brown to purple shades.
Crops in the initial stage of growth, for example, cereals in the 1st - 2nd leaf stage, have very low cover per square metre, so that the nitrogen uptake and thus biomass are low.
In spring, the coverage is usually 60 - 80 % (estimated N-uptake of 20 - 35 kg N/ha) and the colours on the map are different shades of brown. Brown colours represent little to no growth in the field.
Each brown shade represents a certain growth range. If the growth on the field is only within one growth range, the field will only be coloured in this shade of brown. But that brown colour will still imply growth.