A subcarrier-pairing entropy loading (SubP-EL) scheme with fairly low complexity is proposed for digital subcarrier-multiplexing (SCM) systems with colored signal-to-noise ratio (SNR) distributions. With the constraint of the target entropy, SubP-EL iteratively optimizes the entropy of subcarriers in pair. After convergence, SubP-EL can approach the optimal performance which is evaluated in simulations and experiments by comparison with the brute-force search method. Meanwhile the complexity of SubP-EL is significantly reduced compared with the brute-force search. In particular, in an 8-subcarrier system with five different SNRs, the complexity of SubP-EL is reduced by approximately a factor of 764, 95 and 13 with the entropy granularities of 0.05 bits/2D-symbol, 0.1 bits/2D-symbol and 0.2 bits/2D-symbol, respectively. The performance of SubP-EL is evaluated in simulations and experiments. In the experiments with 345 km fiber transmissions, the average NGMI gain of SubP-EL over the system without entropy loading is 0.0286 for different optical filtering cases.