Yellow wire +12 volt
Red wire +5 volt
Orange wire +3.3 volts
All go to motherboard and get distributed and regulated. Some go out directly to other hardware like drives and GPU's.
Look up your PSU at CM's website for its +12 volt rating if you can not read it of your sticker.
What is the model number of your PSU? On one of the sides of the PSU there is a label that will list the amps available on the 3.3V, 5V, and 12V rails. A lot of times, there will also be a max combined voltage listing for a combination of the rails (i.e. 3.3V and 5V max, 12V1 and 12V2 max, or max combined watts for 3.3V 5V and 12V).
The single largest loads on modern PSUs are the CPU and the GPU; both the CPU and GPU pull from the 12V rail.
6-pin PCIe power connector is suppose to be able to supply up to 75 Watts or 6.25 Amps.
(6+2)-pin or 8-pin PCIe power connector is suppose to be able to supply up to 150 Watts or 12.5 Amps.
The Cooler Master 350 Watt models usually specified a +12V rating of 22 or 23 Amps depending on the model. I personally wouldn't trust what Cooler Master has specified on the label unless it's been shown that it can deliver it based on reputable test reviews.
so if i connect my gpu via pcie cable then will it pull the load from +12v rail?
Yes. The graphics card gets the majority of its power from the +12V rail. There is a small amount of power drawn from the +3.3V rail through the PCIe slot (i.e. 9.9 Watts maximum according to PCIe power specifications).