LED lighting is increasingly being used not only for illuminating the human environment, but also for plant growth. Improved energy efficiency is only one of the many advantages of this technology. For example, luminaries can be placed closer to the plants, because they radiate less heat than gas-discharge lamps. Tunable light sources can also be realized using color LEDs, which enables horticulturists to optimize the intensity and spectrum of the light to best suit the plants based on their species and life cycle. In this project, a broad spectrum, arbitrarily tunable luminary is designed as part of an indoor plant growth system. To define the required intensity and wavelength of light, a basic insight in biology was taken. Optical calculations are used to do conversions between luminous flux and photon flux, and to create a spectrum visualization tool which is then utilized to determine the type and number of LEDs being used. The device incorporates 176 pieces of 3 [W] LEDs with 10 different colors. The core part of this project is the design of a current controller circuit, which involves selecting the circuit topology, the controller IC and additional passive components. For each color, an open-loop peak current controlled buck converter is designed with 200 [kHz] nominal frequency and constant off-time. The total input power of the system is almost 400 [W]. After checking the operation of the circuit with simulations, a printed circuit board (PCB) was designed, focusing on the optimal dissipation of generated heat. The device was manufactured, and went through tests and measurements successfully. The system had been operating for about a month, 8-10 hours a day without any signs of malfunction or overheating. Based on that, the project appeared to be successful, and further development also seems possible.