Does anyone have any hints on how to tell what tone will be taken by 差 ?
For example
时差 cha is first tone
差不多 cha is fourth tone.

Does it relate to shades of meaning, or part of speech, or the tone of the other characters in the compound word, or is it just random?

Rachael.

Ditto for 为
Rachael.

noun --> first tone
verb or adjective --> fourth tone


[chà] differ from; fall short of (verb)
我们离父母的要求还差得远。

[chà] not up to standard; poor; weak; inferior (adj.)
成绩不算太差。The results are by no means poor.

[chā] difference; dissimilarity (noun)
本质差别 an essential distinction

[chāi] send on an errand; dispatch
出差

[cī] as in 参差 (uneven)
同学们的汉语水平参差不齐。


From: http://www.nciku.com/search/zh/detail/%E5%B7%AE/4133

and for 为

[wéi] to be, act as (verb)
他选朋友为他的代言人。He chose his friend to act as his spokesman.
用兵之道，攻心为上，攻城为下。In war, to attack the mind is first, to attack a city second.

[wèi] for, in the interest of
不为名，不为利
seek neither fame nor gain

Brilliant!

Thanks a lot 穆儿.

Rachael.