It's probably multiplying the colours. That's pretty typical for a parameter like this.

Each colour channel is interpreted as a value from 0 to 1, then multiplied with the corresponding colour channel of the tint colour. So:

 _______________ times red (1, 0, 0) =
 white (1, 1, 1) = red (1, 0, 0)
 grey (0.5, 0.5, 0.5) = dark red (0.5, 0, 0)
 black (0, 0, 0) = black (0, 0, 0)

Under this scheme, pure blue (0, 0, 1) tinted with pure red (1, 0, 0) will turn black. But if you use intermediate blue & red colours (ones with some non-zero values in the other two channels) you may get a dark purple.

Here's an example of using "multiply" blending to tint colours, via [this tutorial][1]
![Multiplication tinting example][2]


 [1]: http://design.tutsplus.com/tutorials/quick-tip-using-layers-and-blend-modes-in-inkscape--vector-25907
 [2]: https://i.sstatic.net/U0Dye.png