Tyranids are deceptively simple to paint. You don't even need to highlight like the GW pictures. Best and simplest way to paint a fully organic army like nids is to simply wash the hell outta them. Pick bright colors and give it a Devlan Mud wash. The two main colors on most tyranids are gonna be the flesh/skin and the carapace. If it looks dull to you, add some more color where possible. The tongue, eyes, teeth, claws, those "tendon" looking things on their arms. Maybe some bright green for venom sack or whatever. There's only a few things you can do to a nid to give is color. For great color ideas check out
www.coolminiornot.com
I don't agree, some of the best bugs I have seen where very colorful, Mine included.
You just have to look at it like a color theory problem for the most part. Yes you want it to look kind of natural but at the same time it does have to be dull drab colors, as is evident By even just the hive fleet Leviathan in the book with the whites and pinks and purples. Its all about how you bring the colors together. More of some colors less of others. If you want to spice a bug model up you can focus on their backs, the carapace can be really over done and look awesome. Try looking at pictures of beetle shells for inspiration on designs and colors you could use with that dark brown you said you use.
You can also make use of triad colors. Were you look at a color wheel and see groupings of three or four colors, then just variant those with shades in dull and boldness.
If you look at mine I run a green scheme and to really make them pop on the battle field I put dots of red or details as it were because when your eye sees green and red of the same intensity it causes a special optical reaction called symotanious (sp?) color. And the colors begin to vibrate where they touch. It brings life into flat objects.
Other ways are layering if you don't want the "that hurts my eyes to look at for a long time" factor. By just building up colors with washes and highlighting you can get great effects. The best are when you use two completely different colors on top of one another. Leviathan is a good example with that white and pink, so simple yet so powerful when put on top of one another.
And all this takes time to learn, lots of experimentation. I studied color for a full year and still getting the hang of it. Color is hard to work with effectively, but nothing is more visually appealing as good color control.
So to make a long winded statement shorter, look at beetles and other real life bugs, see how their shells use color, really study all the layers of color in their shiny bodies and try to replicate something similar to make you models stand out on the battle field.