為什麼溶岩是橙色的?

Yellow lava is hotter than red lava. You will see that yellowish-white lavas are about 1200 deg C, but that dark red ones are only about 700 deg C.
If it was a lot hotter, it would be white or blue. But in real world the temperature of lava usually never higher than 1450 deg C, so the colour we usually see is ranging from red to orange to yellow.
It is about black body radiation, as the temperature of lava is ranging from 1000K to 1500K, due to the fact that different composite of lava have different melting point, lava with more mangensium compound will have the higher temperature.

圖片參考：http://www.egglescliffe.org.uk/physics/astronomy/blackbody/Image21b.gif

From the graph, you can see that the peak intensity of spectrum occur for 1000K is about 300um, while 1500K is 200um which is infra-red radiation and outside of the visible range, so only some visible light of the red end can be seen. For 1000K, you can see visible range spectrum have only red, so lava is dark red in colour. But for 1500K, you can see visible range spectrum have red + orange + yellow, so lava is orange or yellow in colour.
I hope this can help with your understanding. =)

2007-12-01 20:07:38 補充：
If you want to know more about lava, you can see another question answered by me,http://hk.knowledge.yahoo.com/question/?qid=7007082600350=)

因為觀察者戴了橙色濾鏡