Every resistor has colored bands. Those bands and colors are not random at all, they help to identify the specifications of the resistor.
Here is how you can see the value of a resistor depending on its colored bands.
Every color represents a different number.
Here is the table of all the colors and numbers:
Counting from right to left, the second color is the multiplier. Digits of the first colors must be multiplied with the number of this color.
And the last color is the tolerance. Tolerance is the precision of the resistor and it is a percentage.
There are a lot of programs that can calculate the value of a resistor but if you do not have access to the Internet and you need to know a certain value you can use the table from the next figure.
1.Yellow and green represent 4 and respectively 5. They represent the first and second digits, so you will have 45. The third band is orange. As a multiplier, it is x 1000, so you will calculate 45 x 1000 thus, the resistance is 45,000 Ω. The forth band, silver, represents the tolerance, so the final expression of the resistance is 45,000 +/- 10%Ω. 2. A resistor colored Orange-Orange-Black-Brown-Violet would be 3.3 k$\Omega$ with a tolerance of +/- 0.1%.