There is a slightly more time consuming way of getting straight lines, which involves making a fill layer, then Transforming it with 'stretch' and 'lock rotation on'. Making a few different layers, and messing with them achieves this image. You can drag your white layer to narrow your coloured line as much as you like. I used Blend Mode Multiply on the top layer to get the merged colours.

If you want to use Transforming to narrow the line, put a dot away from it a distance above and below to form a bigger selection so your fingers have room to get in there and maneuver. Rub those marks out later. These lines can be duplicated, transformed and dragged wherever you like.

A good practice one is to try and get a horizon line with two separate layers, just drag the top layer down. BUT, remember to Lock Rotation so it doesn't wiggle off the horizontal!

Click image for larger version. 

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The mushiness of the colours is just compression for uploading.