Routing and Spectrum Allocation (RSA) is one of the central problems in modern optical networks. In this setting, also called Flexible Grid, we want to find paths and allocate non-overlapping frequency slots for as many demands as possible. As opposed to the Routing and Wavelength Assignment (RWA) problem, where all frequency slots are of the same width, demands in Flexible Grids may require slots of different sizes, what makes the original NP-hard problem even more challenging. In this paper, we consider Spectrum Allocation (when routing is known), develop an efficient greedy algorithm based on "degree of saturation"-coloring, and by means of Constraint Programming, we show that the greedy algorithm is almost optimal even for hard real-world instances.