iam building a hell cat.
and i painted the bottom of the hell cat gray.
but i used tape for the line and when i took the tape off there is a line
how can i paint 2 kinds of colors with out geting that line
thanks

I assume you mean you want a soft line, where as tape gives a hard line. Use an airbrush, that's the easiest way. Another way is to hold a piece of cardboard a few centimeters away from the model as you spray from a can, this will give a soft edge.

If you mean the hard demarcation line between the two colors, there's several ways to do it based on how dangerous you want to live.

1. Mask the area where the separation of the colors is with some type of paper such as post-it note or note cards that are held off the surface just a bit by putty so the paper isn't resting on the surface of the model.
2. Use putty (I use blue-tac) to shape the curvature of the demarcation line and spray the paint perpindicular to the putty to get a nice graduated demarcation between the colors.
3. My personal favorite-freehand it with an airbrush. This takes practice and a little bit of courage your first time but once you do it you'll probably never try another way. Practice not only the airbrushing technique but practice different paint/thinner ratios as well. The paint/thinner ratio plays a big factor on how well you can make the line without too much or no splatter of paint.

Here are examples of methods two and three above.

Putty Outline-This is a Corsair I built and painted the tri-color camo scheme with the putty method described in number two above. Not hard to do but took some practice.

Freehand-This is a Gloster Gladiator I built and first attempt at freehand camo. Really easy after practicing and the only way I'll paint camo from now on. This is your typical camo but the method is the same if painting two colors and you want a soft demarcation line between the colors.

For this I use UHU - Tac Putty. I draw the line with it and airbrush the secon colour.

I think you're talking about a ridge of paint at the demarcation point. That's from laying down too much paint. You're using an aerosol spray can or brush painting, right? If you're using a can, move further back from the model and use lighter coats (move the can quickly). If you're brush painting, freehand the demarcation.
The best, though it is expensive, way to paint is to use an airbrush.

i wanna thank u guys..... for helping me i will try free hand line and the air bush,
i just started to build agian. and all my planes had bad lines,
and thnaks tin can. that plane looked great

If you're talking about paint bleeding under your tape then here's the trick. Paint your first colour, mask, then give the demarcation line a little shot of clear. This will run under any high spots in the tape and prevent your next colour from bleeding into your first colour.