Currently all new decorated parts have transparent backgrounds, so that the parts can be recoloured, however older parts weren't made this way; they need to be updated. This process can be very time consuming, which is why not all parts have been converted yet. Older parts can still be recoloured; there is an algorithm which changes everything everything in the base colour of the part to your selected colour. Some color values, like Medium Lilac and Flame Yellowish Orange, have been changed since these parts were created to make them more realistic. Because the colour in the part is different to the value Mecabricks is trying to replace, the colour isn't changed.

In short, the parts don't change colour because their textures don't interact properly with the new parts system. Eventually they will be updated, so this issue is only temporary.

thanks for your reply