The two main wires coming from the truck are yellow, and blue with a red stripe.
The yellow is switched by the ignition switch, and is what you want for your main power.
The blue with red stripe wire is controlled by the light switch, and is what powers the light(s) in the radio, such as the one which illuminates the dial. Only use this wire if your new stereo has an illumination input. Then the radio lights will dim with the rest of the dash lights when you adjust with the light switch.

There is no actual ground wire for the radio, other than the speaker ground wire. The radios mostly get their ground from the antenna and from metal to metal mounting connections. The new stereo's ground wires can be connected to a good ground under the dash. Usually the ground wires will have a screw-down terminal on them, so they can be attached to a good grounding point under the dash, with the screw.
You will probably need to check the speakers themselves, to see which wire is which.



Now if we're talking about wires coming out of the radio, yellow is usually the power input from the ignition switch, which only receives current when the switch is on.
Red is typically the constant power input which goes straight to the battery. This saves your station presets and clock and stuff like that.
And blue is typically a power output for a power antenna or amplifier.
But that should all be in your instructions. But if you didn't get any instructions, try googling the new stereo's model number, and you might find the instructions, which should include a pin-out diagram which tells you what each wire on the radio is for.