Something I felt was worth drawing attention to- if you look at the "glow" around True Excalibur and True Night's Edge, you'll notice that the two sides of it reflects the color scheme of the biome to blade represents- for Hallow there's light blue (the sky and grass) and pink (the primary color), and for Corruption there's lime green (cursed flames) and purple (pretty much everything else). As well, the Terra Blade sort-of reflects this as well (this might be a bit stretching it), as the blade is colored blue and lime green- the two left colors on both True blades.
Might be something worth keeping in mind.
EDIT: Also another thing, getting the evil swords might be hard or impossible to get in a single world due to the limited number of Muramasa's- there's no guarantee you'll get the three you need without going to another world.
As for Terra's color-sheme, I think it was mostly intended to have a cool shading maybe representing day and night, the green is clearly just to make it feel "Terraria-like". I managed to incorporate this with minimal effort to instead represent the blue playthrough, hence why the other two blades will be in a gradient from red to green/yellow, and faint green to green/yellow.
Regarding the true swords; The colors embedded in each true version are indeed the biome's imbuement/spell ingredient's color, whilst the evil swords have their biome-color incorporated as well into the edge itself. At least that's how I choose to view True Excalibur, since the crystals can be both blue or pink. It makes sense that it would have two different colors, even as a "good" true-sword due to the multi-colored nature of the biome itself.
About that last observation though- you got me. I really hadn't thought about that... This calls for a solution immediately! I'm guessing that either there could be an enemy in the easy-mode dungeon dropping them, but that would require either as a rare drop or a rare spawn, both of which surrenders the task to RNG.
-Or how about this:
crafting 15 souls of Fright (red) with a broken hero-sword for True Wrath of The Gods,
crafting 15 souls of Sight (green) with a broken hero sword for True Brink of Deformity,
crafting 15 sould of Might (blue) with a broken hero sword for True Night's Edge.
I think it would be the best alterantive. You'll need either Excal or Night to defeat the mechanical bosses, and after the mechanical bosses + Eclipse you can choose to make any true sword you like. The Souls should of course be replaceable with the alts.
-So instead of removing impurities in the process through recrafting you just bind the according essence with a broken hero sword and turn it into its purest form directly.