Colour is temperature
Why Are Some Stars Red and Others Blue?
A star's colour is not decoration. It is a thermometer you can read from a hundred light years away.
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Step 1 of 4
Anything hot enough glows.
Heat a poker in a fire and it goes dull red, then orange, then yellow-white. It is not catching fire — the heat alone is making it give off light.
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Step 2 of 4
The hotter it gets, the bluer the light.
Colder glowing things put out mostly red light. As the temperature climbs, the light shifts through orange and yellow towards blue. Red is the cool end, blue the hot end.
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Step 3 of 4
So a colour is a temperature you can measure from here.
A thermometer that works from a hundred light years away, and needs no batteries. You cannot put a thermometer in a star. You do not need one. Split its light apart and the colour balance gives you the surface temperature directly.
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Step 4 of 4
Red does not mean small and blue does not mean big.
Colour tells you temperature and nothing else. Some red stars are enormous and cool; some white ones are tiny and ferociously hot. Size is a separate question.
Our Sun is yellow-white and sits in the middle of the range, which is why it is described as an ordinary star.
The short version
A star's colour is its surface temperature: red is the cool end, blue-white the hot end, and it says nothing at all about size.
Try it yourself
On a clear night find Betelgeuse in Orion's shoulder and Rigel in its foot. One is clearly orange-red, the other blue-white.