If the ETo index has also other inputs like precipitation, I believe that you can perform a comparison with SPI to assess the degree in which precipitation accounts for drought development over Colombia. You can also compute the ETo index based on time scales like the SPI, in order to find coherent results.
Some examples in these publications
Article Comparison of seven meteorological indices for drought monit...
Article Comparison of multi-monthly rainfall-based drought severity ...
You can compute the drought indices that they have ETo as an input. Then calculate the result of that index with SPI, as well you can depict the changes of SPI, the index, and ETo in a graph and trace the results, overall.
For computing different indices as well the indices consist of ETo in its calculation as an input, you can see the following link: