Think in terms of warm colors and cool colors. Warm colors have red in their makeup: brown has red in it. Grey doesn't. Cool colors have blue or green as an undertone.

Wear cools with cool colors and warms with warm colors.

Don't mix patterns. If you're wearing a plaid shirt, don't wear another plaid sweater over it, unless they share colors and the sweater's pattern is both large and limited to only one part of the sweater, and only the collar of the shirt can be seen.

Also think in terms of tone. You can wear just about anything with dark pants, but stick to lighter colored shirts with light pants. The lightest colored khakis are really summer pants anyhow, so you wouldn't want to wear even a light colored sweater with them.

As far as your grey outer coat with grey pants and a dress shirt, probably yes, but I'd need to see the style. If the coat is very casual-looking and so are the pants, then probably not.

The nice thing about both grey and khaki is that LOTS of colors can go well with them.

The one thing to avoid with the grey pants, especially the ones that are almost black, is wearing a shirt that is also grey or close to black. If you wear two items that are black, or close to it, you want to make sure they are the exact same shade, or else it can come across looking strange. That being said though, you could wear a lighter grey pair of pants with a black shirt and it will look very good! Brown is another color that you may want to avoid with grey or black pants; one rule of thumb I've learned thanks to my girlfriend is that black and brown together is generally a no no.

With the khaki pants, you have all kinds of options. Variations of blue, red, green, white, even purple will all look great with khaki. I'm a big fan of vertical striped shirts, so if you have some of those, or are looking to buy some, its a nice way to add a few different colors to contrast with the 'plain' khaki.

I think the grey jacket and pants would look great together, especially if you wore them with a white or off-white dress shirt, with the top button unbuttoned.

Well, my number one rule is Khakis go with just about EVERYTHING! Khakis are a bit less dressy than black or blue pants. I tell my Husband that if his shirts are a gray or black hue then wear these with black pants, those shirts with blue hues should go with blue pants, and the others should go with khakis. This is an easy way to know that you are dressing well! Another important thing to remember is NEVER, EVER wear blue socks with black pants or vice versa. Her are some links to help you out! Good luck!

http://www.askmen.com/fashion/fashiontip_250/256_fashion_advice.html
http://mensfashion.about.com/od/wardrobebasics/Wardrobe_Basics.htm
http://mensfashion.about.com/od/wardrobebasics/a/6rules.htm