Thanks for playing, and thanks for the in depth report! It's interesting to see someone else's creativity, and confirms how I feel about the potential of this game. It's obviously very bugged in its current state, you especially seemed to get hit by the attachment points getting into a bad state (they're supposed to disappear after a piece is attached). I have some plans to address Unity's so-called "fixed" joints acting as springs because the current state of the physics is absolutely unusable for anything more than 10 minutes of goofy entertainment. I really don't want it to be like Elastomania.
To clarify the strange colours, they are supposed to show when the spot you are attaching to is ambiguous. In the case of the hinge, you can either attach to the hinge or attach to the same point the hinge is attached to, which would alter the behaviour of the new piece. The colours are chosen at random in a way that maximizes contrast, and the player needs to select one of the coloured groups of parts to attach the new part to that group. Not selecting one puts it into a bad state as I currently don't have safeguards. I also need to make the UX of it way more intuitive, as with the entire rest of the game.
As for the keybindings, you can change them for each part simply by clicking on the letter in the bottom part of the UI. This UI needs work too, I'm currently thinking of a vertical controls menu on the right hand side, opposite the parts list.
You're the second person to suggest being able to place a part repeatedly, so that will definitely happen. I also want to make a system to store pre-saved groups of parts that can all be placed together. Any added parts will have to fit into a pre-defined build area, preventing both placing inside the level geometry and any advantage placing the robot closer to the goal.
A long way to go still, but you've given me a bit of inspiration. Thanks for your efforts.