I combined two translucent eSun brand PLA filaments. The blue is "glass light blue" and the red is "glass watermelon red." I appear to have achieved an even mix of the two, with a translucent purple color.
I used an extrusion multiplier of 1.4. I will probably go slightly higher next time, maybe 1.45.
edit: I added a photo of the melted tip of the filament after I pulled it out of the hot end. It's interesting how the different materials form a striped/banded result that is perpendicular to the cross-sectional pattern of the incoming filament. I postulate that inside the hot end, the filament is buckling and doing a zig-zag motion as it is fed into the melt zone. Because the filament is significantly thinner (when you print it, Z-axis direction) than it is wide (X-Y direction when you print it) it would tend to buckle in such a way as to create the banded pattern seen in the photo.
Glass light blue, Glass watermelon red