changing from a color to another color in continuing way

Select your line and go to "Fill & Stroke" (in the Object menu or with CTRL SHIFT F). The second tab (Stroke) shows (in the top) 7 options with different icons : a red cross (no stroke), a flat color (flat color ;) ), and a gradient. Choose this one. The default gradient should be linear (which is what you want) and black to transparent. Click the "Edit" button just under the gradient sample. You want the first stop (=left color) to be blue, and opaque so choose your blue color the way you feel comfortable, and be sure to have the "A" slider ("Alpha" = transparency) all to the right. Then your 2nd stop (=right color) to be red, so in the top, select your 2nd stop, set the "A" all to the right (by default, it should be transparent so A should be zero), the choose your red color. Once done, you can close your window and (hopefully) you've got your gradient !
regards