I 've just installed a new thermocouple on my wall furnace but there's a "YELLOW FLAME" mixed in the BLUE FLAME" what do this means?

If your pilot light is on but has a weak, yellow flame, it means that it is not working well enough.
If your pilot light goes out numerous times throughout the year, it could mean that you have a problem that needs the attention of a professionalHVAC contractor. Some of the reasons your light may keep going out include:
A bad or loose thermocouple
A bad gas valve
A poor pilot flame - low gas pressure or blocked orifice
High winds or downdraft
A badly cracked heat exchanger
Improper venting
Flue or chimney problems
The most common reason for a pilot light to go out often is a nearby draft; you should rule this out as the cause by simply looking around to see if there is a new draft in the area of your system.
If your pilot light is on but has a weak, yellow flame, it means that it is not working well enough. A pilot light should be blue with the tip of the flame having just a tinge of yellow — a sign that there is enough heat to warm your home. With a yellow flame, the furnace will not get hot enough to heat the thermocouple to the point where it triggersthe gas valve to open. If you have a propane flame, it should have a bluish-green flame with a tinge of yellow at the tip.
The first step in troubleshooting your weak pilot flame is to rule out whether there is a draft that is weakening the flame. If this is not the problem, then you may need to adjust the flame. Follow safety precautions and your system's manufacturer's instructions for adjusting the flame. Usually this is pretty simple and you just need to turn a small screw. If this does not help, the problem could be a dirty pilot tube. For this, you should turn to an HVAC contractor — or if you feel skilled enough (and know all of the safety precautionstoo), then you can take a needle or small nail and gently clean the tube.
Hope it helps! :)