Watch the Flat 6th: Alt. Dom. Sound online guitar lesson by Kenny Wessel from Shades of Jazz
Now, to get a more colorful altered dominant sound, we can use the same altered pentatonic mode (major b6 pentatonic) and use it over a different chord. We'll use the major b6 pentatonic built a minor 6th up from the root of the altered dominant chord to get a really interesting and useful altered dominant scale choice. For example, over a G7#9b13 chord, we can use the Eb major b6 pentatonic scale. The notes we get are: Eb, F, G, Bb, B --- over a G7 those notes give us these tensions: b13, b7, root, #9, and 3. So we get the tritone (3 and b7) and a couple of altered tensions - b13 and #9, in addition to the root.
In addition to being an interesting sound in itself, the major b6 pentatonic has a bit of an exotic sound - notice that there is a major third interval in the scale (from the B to the Eb); this scale is very useful as a dominant scale.