say for instance you are in the middle of the sahara desert wearing a full robe with hood. a full white robe will reflect most of the light and therefore heat up less however more solar radiation penetrates the white so what you gain in light reflection you lose out on in radiation penetration. a full black robe will absorb more light and therefore heat up more but will block out more solar radiation. also the heat absorbed by the clothing is not all transfered to your body. loose fitting clothing is key to this.
in the end, both are about equal in terms of protection.
as for your second question, color does not reflect heat. a materials color has to do with what colors are reflected in sunlight. this affects the energy change into what we know as heat when the rays impact us instead of being reflected.
putting clothes around a computer acts as an insulator due to the thermal properties of cloth. this is completely seperate than color. the heat from a computer is not coming from light therefore color does not matter.
what this means is that the percentage of heat which normally radiates from the case would be stopped causing internal temperatures to rise. this is a bad thing.
the only case where it would help is if you actually had an air conditioned case. thermal insulation would keep the inside colder then the outside of the thermal insulator. however this causes condensation and as we all know water is bad for electronics.