Yup, basically you can take a train from Retiro Station to Tigre for 1,60 pesos (ridiculously cheap), and then from Tigre you can get a roundtrip ticket to Colonia for 260 pesos.
The train from Retiro Station to Tigre takes an hour, not sure how long the boat from Tigre to Colonia takes, but I don't think it would be that long.