Red "-": Private
Blue "~": Package
Yellow "#": Protected
Green "+": Public
Nothing: Undefined (The difference between "public" and "undefined" wasn't clear in the UML2.4 spec., but I think they have updated that in UML2.5)
I don't know. I've never seen this overlay before and I cannot reproduce it.
Also, it seems that some of your properties also have a yellow "-" in addition to the red one. I've never seen this one either.
Do you use a specific profile? Profiles can provide custom images. For example, the arrow on your example might mean that the direction of the property is "in" (SysML has a similar icon for flowproperties with direction = in)
Sorry, missed replying to this. I don't think it contributes any icons, but it does make heavy use of UML generalisations, subsets and redefines, which are pretty unusual in UML modelling - maybe that is what the icons are for.