[Peter Knutsen]

Okay, so far so good.

I've got Sense values for Vision, Hearing, Taste, Smell and Touch. Exotic beings may also have different Senses, such as the Life Sense of many (if not all) Undead, and Danger Sense is (or can/should be viewed as) a Sense. Same with Senes Holy/unHoly.

That works fine. Strength of stimuli, you determine final RD, then you roll to see whether X is noticed or not.

The problem is, I've opted to have Night Vision and Colour Vision as separate Senses. And that's not too elegant. It means that normall you roll for the Sense of Vision, but in low-light conditions you roll for Night Vision instead. If you for some reason need to tell certain colours apart, e.g. determine whether a particular clothes item for sale is or is not your girlfriend's favourite colour, then you roll for Colour Vision.

Also, the Colour Vision Sense value affects the character's Aptitude for some Skills, e.g. many artistic Skills. Better Colour Vision means they're easier to learn, worse Colour Vision means they're harder to learn. Some scientific Skills are also influenced, such as Chemistry (and Physics, but not very much), and of course Alchemy, and ideally Blacksmithing and certain other metalworking Skills to. (Night Vision doesn't currently affect any Skill Aptitudes, and I see no particular reason to change that.)

Colour Vision is abstract. We don't know which colours it is someone with a +1 RD penalty to Colour Vision can't discern between. It's probably red and green, but we don't know, and we don't need to know. If it's a +2 RD or +3 RD penalty then it's probably something fairly exotic, and if it's much higher than that then it's effectively complete Colour Blindness. likewise, a -1 RD bonus is probably tetrachromaticism but maybe it's something else, and -2 RD (if possible for Humans - jury is still out on that one...) could be something like pentachromatism, or could be something else.

I don't much like the Colour/Night Vision as separate Senses thing, but I find it hard to come up with a better mechanic.

I think the way forward might be some kind of secondary modifier that modifies the primary modifier, but that sounds completely horrible to me. I like to have as much as possible pre-calculated before game start, so that in-play resolution can be lightning fast, and I find it difficult to envision a subsystem where a modifier modifies another modifier that won't slow down play by a lot. Something multi-layered is fine when it's handled by the spreadsheet during character creation, and printed in ready-to-use form on the character sheet. But during play it's a huge cost to search-and-handling and should be avoided when possible.