I'm assuming you're fishing too fast if you can't get hit on a senko. If you have the patience, I've found the best way to fish a senko is a weightless t-rig. Cast it to a specific target, let the bait fall to the bottom, and let it sit for another 10-15 seconds. Give it a little hop, let it sit for a while again. If you're bank fishing do this all the way back to shore, out of a boat I'd reel in after the third hop and cast to the next target. Fishing large grass flats, dead sticking the bait to where a cast may take a full minute or more is very effective. I personally get bored of this after a grand total of around three casts and just twitch it like a fluke. I know people will use a 1/4 ounce weight to pitch senkos at lily pads and have success. Senkos also make a great skipping bait, weedless and the bait will react the same regardless of which side hits the water first due to the round shape.
When tournament fishing, they make a good lunch time bait. Just throw out a senko over a shallow flat, let the wind slowly drift you over the flat towing the senko behind you. Hold your rod in one hand and a sandwich in the other. You'd be surprised how often you'll get a fish doing this.
In small rivers/big creeks with slow currents I've often found a senko on a carolina rig to be pretty useful. Tie on a 2-3 foot leader and a 3/8 ounce weight and you can keep the current from putting too much slack in your line and keep the senko near the bottom. The leader provides it playroom to act as if its weightless until the current pulls the senko tight with the weight.
Colors don't really need to be anything crazy. Watermelon red and black/blue laminate will work almost every time.