Here are your two main choices for avoiding a loop.
Method one: match the elements. You need to make the LHS and the RHS have the same number of elements (and they need to be in the required order). Here is one easy way to do that:
a(idx,:) = repmat([255,100,0],numel(idx),1)
Tested:
>> idx = [1;3];
>> vec = [255,100,0];
>> a = [245,255,255;254,252,255;251,250,239]
a =
245 255 255
254 252 255
251 250 239
>> a(idx,:) = repmat(vec,numel(idx),1)
a =
255 100 0
254 252 255
255 100 0
Method two: use scalars. Treat each column individually and assign scalar values. This can be a viable solution if you working with a fixed, small number of columns (e.g. RGB channels):
>> a = [245,255,255;254,252,255;251,250,239]
a =
245 255 255
254 252 255
251 250 239
>> idx = [1;3];
>> a(idx,1) = vec(1);
>> a(idx,2) = vec(2);
>> a(idx,3) = vec(3)
a =
255 100 0
254 252 255
255 100 0
This can also be trivially looped ( I am sure that you can see how).
