  Given a set of $n \geq 1$ unit disk robots in the Euclidean plane, we
consider the fundamental problem of providing mutual visibility to them: the
robots must reposition themselves to reach a configuration where they all see
each other. This problem arises under obstructed visibility, where a robot
cannot see another robot if there is a third robot on the straight line segment
between them. This problem was solved by Sharma et al. [G. Sharma, R. Alsaedi,
C. Busch, and S. Mukhopadhyay. The complete visibility problem for fat robots
with lights. In Proceedings of the 19th International Conference on Distributed
Computing and Networking, pages 1-4, 2018.] in the luminous robots model, where
each robot is equipped with an externally visible light that can assume colors
from a fixed set of colors, using 9 colors and $O(n)$ rounds. In this work, we
present an algorithm that requires only 2 colors and $O(n)$ rounds. The number
of colors is optimal since at least two colors are required for point robots
[G.A. Di Luna, P. Flocchini, S.G. Chaudhuri, F. Poloni, N. Santoro, and G.
Viglietta. Mutual visibility by luminous robots without collisions. Information
and Computation, 254:392-418, 2017.].
